Well, I'll start my review by stating this is my first smartphone, so I can't compare to any I've actually owned before, though I did look at a LOT before deciding on this phone. It's only been out a short while and so I've not had it very long, but I've had a play with enough of it to write a rounded review.
First looks:
No matter how much we like to say it's how it works that matters, I doubt looks ever are not a criteria for buying a phone like this. And I must say, the Galaxy S2 does not fall down. It measures a very sleek 125.3 x 66.1 x 8.5mm - making it as far as I can tell, the thinnest smartphone currently on the market. This is measured to the thinnest point however, it is around 10mm at the thickest part (I don't have an accurate enough tape measure to tell you the exact thickness at the thickest part!). It weighs in at approx 116g, making it also quite a light phone.
The screen is 4.3 inches (diagonally) so a fair bit larger than many smartphones on the market - this is a love or hate thing, I wanted a big screen though. It currently only appears to be available in black - the black looks very sleek and very stylish. The casing is plastic but appears hard enough to be still durable. It has a camera on the back, causing the back there to be slightly thicker than the surrounding area, but not by a lot.. The back is also curved to make it ergonomic to hold - this works well for my hands, but I imagine may not be ideal for larger hands. The front is mostly glass, the only breaks in the glass being the earpiece and the home button. It has samsung written below the earpiece.
Most of your buttons are touch buttons on the screen, but at the bottom are 2 more touch buttons always there and a "proper" clicky button - a short press on this takes you home, a long press brings up the option for task manager or recently used apps.
Initial use:
To turn on the phone you hold the button in - it appears to need to be held for a second, presumably to prevent accidental presses. The button is on the right hand side (as you look at the screen) instead of the top like many phones. It has a volume adjust on the left side, a headphone jack on the top and on the bottom is where you plug your charger/usb etc cables in. It boots up very quickly bringing you to the home page.
General use:
This phone has 7 homepages - these are completely customisable with widgets and buttons for your applications, there are space for 12 standard sized touch buttons on each screen (although some widgets will take up more than 1 space). Fixed at the bottom of each home screen are 4 more buttons - for phone, contacts, messaging and applications.
It's all very easy to navigate through, it's very intuitive to use, and even my father (who sees no point in even text messaging on phones!) was able to find his way to anything on it very quickly.
The keyboard is worth a mention - there are 2 you can select, but I'll concentrate on the swype keyboard - this is the standard keyboard when you get your phone.
As well as normal touch typing, you can use the swype functionality - allowing you to run your fingers over the letters of the word you wish to make. I find this very very quick and easy to use, but from reading online this again seems to be a love or hate it issue - if however you hate it, you don't even have to use it with swype keyboard on, or you can change to the second keyboard.
Oh and of course it has the (now common) accelerometer; so as you change your phones orientation, the screen changes the way pages are viewed Some apps do force this to be in one orientation and on the home pages it can only be in portrait, not surprisingly.
At most times (again some apps, mostly games I've found so far) there's a little function which is handy, you scroll down from the top of the screen and another screen appears, from here you can turn on/off: wifi, bluetooth, gps, sound/vibration and auto rotation, this is also where your notifications appear, you can also see ongoing apps on here (such as IMs).
Phoning/texting:
There's only so much can be said about the phoning on a phone, but for what I've used it so far, this has been perfect. The numbers when typing are large, the call quality so far appears great (of course this is network related too!) - no problems whatsoever. There are different ways you can display text messages I believe, but as standard it shows them as a thread of messages, so you can see the last message(s) when typing a new one to somebody you've already texted. Overall it seems to work perfectly well, no problems whatsoever.
Email/other messaging:
The phone initially has an email application and also a dedicated gmail application - I've only got the latter, but for this you simply connect it directly to your gmail account and you can open your account and read your messages in 1 easy click. I cannot check the system for other providers without making an account but I'd imagine it to be similar.
If you use IM there's a pre-installed app for that too - simply called IM, this can be used for Google talk, windows live messenger or yahoo! messenger. There's also social hub which can be used to manage the aforementioned IM accounts as well as your facebook, myspace, twitter and linkedin accounts, plus your email accounts from several providers.
If you prefer there are of course plenty of other apps that can be downloaded to manage your various accounts - facebook and twitter for instance have dedicated apps.
Other apps:
A quick look now at some other apps you'll get initially:
Clock (doesn't really need much explanation, can have several set to different timezones though, cool if you have a lot of friends abroad), calendar (automatically syncs birthdays with your contacts< music, internet, camera, gallery, maps, google search, music hub, readers hug, game hub, a few games, plus some video apps, talk apps, voice apps - too many for me to talk you through in a short review!
Of all your apps, one you'll use a fair bit - at least at first - is the marketplace app.
Android and the Marketplace App:
Android marketplace has thousands of apps, I've read different estimates on how many but there is an app for pretty much anything. Downloading these couldn't be easier - you find the app you want (you can browse or search, easy to navigate), click install and click ok. It'll then download and install for you. There's a lot of apps, from games, Gtunes (really recommended!), various IM managers - to things like the barcode scanner (which I mostly use when bored but I'm sure there's some legitimate use) and various bubble level apps. Some apps are definitely more useful than others, but if you want an app for something - it probably exists. Many of these are free too!
Oh and samsung also has a few apps of their own.
Other bits:
Well now my thoughts that don't fit into a nice heading. I like contacts, for one thing you can take a photo of your contacts for next to their name, and it'll sync with twitter and facebook and probably other things too. Some of the pre-installed apps aren't the best free ones available, I imagine these must be the ones Samsung likes or something.
There's a task manager that allows you to shut down apps that aren't in use, which I love.
The current phone has 16GB of storage, although a 32gb version will be released, BUT you can put up to a 32GB MicroSD card in to expand storage anyway, if 16gb isn't enough.
The processor is a dual core 1.2GHz, it has 1GB of RAM - meaning the phone never slows down. In fact it's probably a bit more than necessary, but it does mean it shouldn't run into problems quickly when new apps etc are released.
Bad things:
There's only a few of these I can think of and all are pretty minor, but here I go...
Ringtones/notifications: There's a good selection of these but honestly.. none I actually liked, they were all the typical cheap cheerful ringtones you get on any phone, I really hoped on a pretty pricey smartphone there would've been a better selection, quality wise.
Capslock on keyboard - it seems hard to get constant caps, I htought at first I was missing something but now I don't think I am. It's a little niggle, but it is there nonetheless.
The lock screen - if left too long the screen goes black and the lock screen appears, to get rid of this you drag it off the side - while handy in a sense, at times I wish it wasn't there as it's another step to get back to what you're doing - that said, it does stop accidental presses again.
And.. that's it for bad points!
As I said to begin with, this is my first smartphone, so I have no real comparison base from long term use, but I find this to be a fantastic phone. It's very very easy to use, I've not had any problems with things working, the settings etc are easy to change - it really does make life easy.
Final note is the price - I believe the RRP on this phone is around £600, maybe £650 - I'm getting different answers when looking, it is available from amazon directly at £539.99 sim free, and of course is being offered on contract by many companies - and more to come soon too. Its a lot of money for a phone, but a lot of phone for the money.
I only bought this recently, so it's obvious I consider this the best of the current smartphones on offer - so well worth a look at least.
The S2: A slim, very clever smartphone. 
