It comes down to how much you value the ease of access to the Kindle Store and a dictionary, but if you've already got a working eBook Reader, then it's quite a big cost to justify, particularly as the wireless access that we'll get is much more feature-restricted than the US Kindle access.
I guess your first step should be to try & get an idea of the books you'd be likely to buy (and possibly the ones you have bought over the past year) and compare their price & availability from the Kindle store vs the various stores selling EPUB titles (Waterstones, WHS, etc - (See my UK-focussed
"Where to get eBooks" article for links). That should help in summarizing the differences as they'd apply to your own usage, anyway.