I know the W810i is really easy to tweak and chance things. As for a "really good camera" You will have to bend towards the K810i or K850i but if you live in an area that requires GSM850Mhz then you won't want to be steering towards the high end K series.

I think most of the SEs cameras are okay. If you really want to venture into dark waters you can do some research on the Sony Ericsson SO905iCS which is NTT DoCoMo's Sony Ericsson slider phone with a 5.1MP Cybershot camera and 3x optical zoom. This phone has a hypersim unlock hack released and there's many people that have it and have said that all the multimedia options have been unlocked (yeah Japanese like to use network re-provisioning schemes sometimes lock users out of the multimedia sections).

As for the W660i/K770i/K550i, I don't know enough about those phones to really say anything about it so I can only shell out information based on what I've tried and know ;) Thats my 2 cents. Hope that helps.