I agree with the fact that all the previous generation Nokia XM edition phones are just a hype. even I've tried 5700 and as people say here it just marketting gimmik.
I've seen the pics clicked by 5610 on many many sites and the camera is nowhere close to k790i. however since i've personally not tested 5610 i can't comment on its music capabilities.
Among the tried and tested, i'd suggest k790i. you will need to flash the acoustic drivers ONLY if you want a louder loudspeaker volume (provided you think its low in first place) and believe me, it makes a hell lotta difference. Do try it out first and then comment! At about 11.5k i think k790i is a steal!
I've said it earlier and I say it again, only the N91 comes close to walkman phones when pure default audio quality is concerned. I haf no experience with N81, so won't comment on it.
I haf no experience with Ultra range of Samsung as well.
A lot of users may think that since I own an SE phone (the ancient W700i completely modded to the core!), I'm supporting it. But believe me you need to spend some time with the walkman range and XM to know the difference. And yes, there is a lotta difference! The walkman platform, since the early dayz, was a mature one which needs li'l change. Among those phones which I hafnt used (N81, 5610 etc.), I've been frank and hafn't commented.