I have both a SonyEricsson T610 (work provided) and my own Nokia 6600. I also have a Nokia bluetooth car kit.
I think the SonyEricsson is a crap phone compared to the Nokia, although it is slightly smaller, the screen, camera, build quality and general usability is nothing like the Nokia. The Nokia is a phone I like using, the SonyEricsson annoys me when I ever I try and do the simplest thing.
The t610 doesn't work well with the Nokia car kit either, but that's understandable I suppose. It doesn't mute the phone's microphone so you get a real echo. I've been waiting months for work to arrange the proper car kit to be fitted.