I have the K750i and love it. Not being byist but buy the K750i. I have had a W810i for 1 day and have to say i hated it. The buttons are all squished together at the top under the screen...if you gets marks on the screen (which you will due to the fact the screen merges with the buttons) you cant clean it properly! The Walkman feature is a gimick (which costs you an extra £80 over the K750i!). If the song data is to long to be displayed on the screen it cuts it off...as it does NOT scroll (unlike on the K750i). The video and photo quality is almost the same...but K750i wins as it was built for photos and video (Have you seen the advert?). W810i photo quality is ok but it does not have as much colour depth or clarity to it as K750i. Another thing to popint out is that the camera is not protected (unlike the K750i camera). The menus are the same except they have now grouped things into 'Phone memory' and 'Memroy Stick memory' menus on the W810i (accesed by right clicking along the way). The only thing i like about the W810i is the bright screen...but if you havent seen it you wont miss it

Finally...the phones feel (as in 'how it is to touch'). The K750i wins again as it feels like a camera (with the camera lens cover) and it has a very cool design. The W810i has a sleek look but it chips easily as it is all plastic and the walkman bits will eventually fall/rub off as they are only glued in

Battery lasts about the same in each phone but it is slightly less in the W810i (about 1 hour cause of the scrren brightness etc...).
Here is a website where you can rate each phone side by side (going by the features). Do not be fooled by its looks. As someone else said...it is a glorified K750i with Walkman (if you can call it that!).
http://www.gsmarena.com/compare.php3...&idPhone1=1090
Overall:
K750i - the best and original!

W810i - glorified copy cat...no where near as good as original. Walkman is a gimick!