In case you don't want to read the full post, here's a summary: Buy one. It's brilliant.
Slightly Extended Version
At the start of the year, I realised (finally) that I needed to lose some weight. I love me gadgets, so this was good timing, as it coincided with the Fitbit Ultra becoming available in the UK.
I didn't have any particular diet in mind. I just decided to eat less and do more walking, and it's working. I've lost nearly 1 stone so far, which I'm quite happy with. Still a way to go, but it's a start.
The one thing that has kept me motivated is the Fitbit Ultra. Being a stats junky, this is just a dream come true for me. It records every step I make (isn't that a song?) and also, thanks to their handy API, all the meal data I enter via the MyFitnessPal iPad and iPhone apps gets synched up to Fitbit. You can enter the meal data using the Fitbit iPhone app or web site, but I find the MyFitnessPal app has a far larger database.
Just like Foursquare, Fitbit gives you badges when you achieve something. For instance, I got my first 250 miles travelled badge this morning, which actually made me feel pretty good, and that motivates you even further.
You can track your weight via the site or the iPhone app, which then shows you a nice (hopefully declining) graph, along with a whole bunch of other useful data (how many calories you can consume for the rest of the day etc).
I opted to pay a little extra for the "personal trainer". This provides you with an ever increasing set of goals for you to achieve each week and helps push you a little further each week. You can manually adjust these if they're too easy or difficult.
I can't really find anything to criticise. The battery lasts for ages - at least a week, usually much longer. The sync works really well. Just leave the USB cable in, and when you walk within range of it, it'll do a sync every 15 minutes or so. The USB unit also doubles as a charger.
You get a handy belt clip and also a comfortable cuff so you can wear it in bed to monitor your sleep.
There's a display on the device itself, so you can check your daily steps, stairs climbed, general activity level etc on the go, without requiring any online access, and the single button can also start/stop a timer so, if you're planning on a walk, just hold down the button for 2 seconds at the start, then do the same at the end and it'll appear as a distinct activity on the site, which you can add additional data to if you wish.
These things are brilliant. They should supply them on the NHS. The cost would be nothing compared to the savings in having healthier people around. It's addictive checking your stats, so there's a real incentive to keep going with the diet and exercise, as you can see the immediate benefit in what you're doing.
As I said in the summary, just go buy one. It's probably the best 80 quid I've ever spent.