I'm soon to be replacing my phone (a Nokia 6230), and am currently thinking of buying the W810i. From
the reviews it seems to be among the best Sony Ericsson have produced so far. I've always been quite weary of them, because I find their interface to be annoyingly slow. But now pretty much all phones are this slow (the 6230 was a nice exception). I have tried it though and it's acceptably fast.
Also, the main breaking point of every SE I've read about and my mates have had experience with has been the joystick not working properly. The W810i doesn't have the same joystick control, so this problem may well be absent too.