I found the battery on my last phone to have depleted to the extent where going to work and coming home 12 hours later often left me with a dead phone, so rather then doing the simple thing and buying a new battery to extend the life span of my then handset, the slightly disappointing K770i was replaced by this.
Currently retailing for around £200 ($240) as a handset in the UK with CarphoneWarehouse (£195.69 on Orange, or £229.95), and various on contract. The deal I got was £30 a month, for unlimited internet, unlimited texts and 600 minutes with the phone for 24 months, effectively getting the phone for £120 when considering my previous deal was the same with out the phone. The phone takes all the old accessories form the previous phone, which included the M2 Memory card, the headphones, the chargers, the USB-Computer connection cable and the speakers set I've got, which was meaning I was certainly going to get a Sony Ericsson. What this one boasts over many of the others (such as the C510i) was a 5 Mega-pixel Cybershot Camera with the "BestPic" function, 126MB internal memory and a rather large 240x320 (pixels) screen. Though not up to the heavy standards of the C905i with it's 8 mega pixel camera, the phone was the natural successor to the k800i and the k750i which were both phones I loved a few years back.

The music player which is one of the keys to any mobile phone that I'll own is also an upgrade on my previous phone and is actually quite reminiscent of the W900i media player which was one of the few great things that that phone actually showed. The media option opens up a menu to show the videos, photos and play the music, and unlike the previous phone I had, this will run in the background with out being temperamental, a brilliant addition to the phone. It's nice to see Sony actually listened to complaints of customers to change the phone so that it's more user friendly to us media nuts.
The phone also manages to have an in-built FM radio for those who prefer to listen to the radio rather than MP3's, the only flaw as far as I can see is the lack of AM functionality in the radio, in the day and age where DAB radio is taking over, it's only a matter of time before a mobile has a built in DAB.
As well as the normal camera the phone also has a camera on the front of the device to act as a camera for video calls, which I've yet to use (I'm never likely due to the over the top price of the service). Which reminds me of the actual purpose of the device, it is a phone (how could I possibly forget) which makes calls, the sound quality, like much of the device is brilliant, it's loud enough to hear everything, and clear. The phone as far as I'm aware hasn't had much signal errors except during the voyage through 1 village where mobile phones seem to he as useful as a chocolate fire-guard. The phones speaker phone function is brilliant and completely un-distorted which is a rarity with the function.
Though for you texters out there, I have a gripe that may put you off, texting with the phone isn't the easiest or the fastest, if you, like me use T9 for your texts the phone occasionally seems relatively unresponsive and slow. Sadly the biggest problem is actually the buttons, which on the face of it seem a little squashed and take a few days of heavy use to get used to it, I'm still finding it easier to send texts off my previous phone though slowly and surely I am changing over in regards to the texts.

What still isn't great is the inbuilt net browser, Sony seem to have been forced to use the Vodafone system for the Vodafone branded handsets, which is a shame but as soon as I got mine I managed to get a variation (3.2) of the Opera browser until I finally managed to get the newest version. I admit again not many folks will use the internet heavily, but those that do need to download Opera to get the most from it as the default browser is relatively limited and lacking.
For those of you with more friends than myself, even you wont be running out of space in the address book, which is capable of holding 7000 numbers (spread over 1000 contacts, oddly) the text inbox size does seem to have increased, but for now I'm not certain, so that may take some time to actually find out about.