Storage, Dimensions, & Use
I love this mp3 player, in my eyes, it truely is one of the best of the bunch. I rate it highly because of the high battery life (~40 hours). I don't use it everyday anymore (I don't walk or cycle as much as I was) but with heavy daily use I still only needed to charge it once a week or so. Now, I literally charge it once every six weeks or so - and I still use it a fair bit, just not every day and not on every journey I take.
The SonicStage software takes a bit of getting used to and it is slow - takes half an hour on average to transfer a few albums - but of course there is always a downside to every benefit. In this case, the atrac3 conversion means the NW HD5 stores not just 20GB of music - but about ~50GB in reality (when compressed). I can store most of my mp3s on this, and of course not everyone wants or needs this, but for me it's a major plus point because I am lazy with updating and switching tracks I currently want to listen to. Instead, it's just there - and easy to pick up old tracks or the new ones.
I don't consider this product to be very large, it's still fairly slim, and easily portable, but for those who want something really really mini it's probably not for you.
Can be used like an external harddrive, too, with no problems

The cover for the charger cable and computer connection is a bit flimsy. Mine is fine because I'm pretty careful but my chlumsy bf managed to rip his off - although to be fair his is almost four years old (they last a long time! neither of us have had any software problems).

Sound quality
Of course, this is really important for music lovers. I consider the sound really good, even with the file compressions, vocals stay crisp and there isn't fuzz unless the original mp3 isn't of a good quality. The headphones provided aren't very good, but I don't consider this a downside as most users buy headphones to their own spec. I personally use the Marshmellows, as they are very comfortable in my ear and I can easily untangle the cords because of the central extension connection in the cable. Also useful if kept in a top jacket pocket - less cord to hide as it can be shortened. The downside of the extension is that when running you may jolt the jack out of the HD5, or the extension may come undone.

Navigation (menus)
I find the front buttons are simple and to the point - not too many of them and it's clear what they do (they're marked). For those who change the volume while it's in their pocket, there's a raised part of the increase volume button so you can tell if you're pressing the right button.
The search is simple enough to understand - search with artist, album, track, genre, new tracks, or playlist. I personally stick with artist or album for the most part and it works well. if you fill the nw5 expect it to run a bit slower than if it were only half full! Scrolling doesn't take too long tho - it speeds up as you scroll a lot.
The second menu (press menu twice) is simple and easy to navigate. Most settings you'll most likely rarely use but they are there. There's options for repeat, play mode (normal playback, one song, shuffle tracks, shuffle items), sound equalisers, avls (no idea what this is for), beep (as far as I can tell all this does is beep when the battery is running low - I just keep it on), you can change the audio output to line out (for speakers), change backlight settings, alter the contrast, invert the screen (default is black background and white text), orient screen (this can be set to auto, so it will change if you hold your mp3 player sideways), language, and there's initialize system.

Accessories
The case it comes with is felt and does not protect the screen very well from scratches. You can buy clear screen protectors to cover the screen (it will appear invisible and you can cut them to size).
There are a lot of cases that appear on ebay searches - from leather to silicon skins. You can also get armband cases that are for this model.
FM transmitters can be bought for using this in your car.