I recently bought the SE K750i Phone. It is a great phone, with a 2 MP camera which actually takes pretty good pictures. However, there were some issues with the phone which caused a minor annoyance:
1. It did not have the "search" function in address book. You can only go to the first character of a name, but cannot actually type in the full name.
2. Even though it had walkman functionality, you cannot use it on airplane.
3. I was having issues with its reception at some places (like my living room), where my old SE z520 was working fine.
So anyways, these things were enough to do something to resolve them. Because of the poor reception at home, I was even considering getting the Nokia 6270 (Nokia has a reputation to have really good reception). On looking up the web for playing around with K750i, I found some links where people installed the W800 firmware on this phone without any problem.
I followed the guide on http://www.esato.com/archive/t.php/t-113981. It uses a software from totalserver.net (which is around 15$) to flash the phone. However, the operation was flawless and within half an hour, I had a perfectly working W800 phone.
From initial looks, W800i is almost the same as K750i. Some people claim that it takes better pictures, and has a longer battery life. I have to verify this yet. But having a searchable address book and better reception alone was totally worth it.
Final Verdict: Sony Ericsson K750i with W800 Firmware ( Highly Recommended ).
Update: There are still major signal issues with Cingular on this phone.