I plan to get one myself. I'm going the extra £40 for the 3G model because then I can download a book whenever I feel like it without having to find a computer first - such as when I'm away in the caravan. With the Sony ones, apparently, you don't get the 3G function - you have to use a computer to download.
There's also the fact that even if you haven't got any money you can still get a fairly wide range of free downloads from Amazon - mainly classic books. No to everyone's taste, I know, but then if you're desperate for a read ...
The downside is that libraries and their suppliers seem to be going for the Sony format, but with the cost of even the paid-for Kindle books being so low, it's probably not worth worrying about.