I've had a W810i for a week and I love it.
The W810i can do the following things that the K750/W800 can not
1. Use Edge Data.
2. Has quad bands, 850/900/1800/1900 .... the W800 does not have 850 and that matters a lot if you're using cingular.
3. Its more or less this year's model vs. last years. If that matters to you at all.
4. I thought the W810 looks better.
If you had a w800 ... I wouldn't upgrade to the w810i. But I like my w810i quite a bit after using it.
head on over to howardforums.com ... they know way more about cell phones than me!