I did some sort of improvised wrap-up of the 5800's sound here:
http://www.head-fi.org/forums/f15/mo...ml#post5643145
I must say, I do appreciate it's sound quite a bit more by now. It's not like it "burns in" or anything, but it does definitely grow on you! It sounds really good for a phone and those situations where one uses a phone for music listening.
I changed from the Sony-Ericsson W810i, which was great fun to listen to – aggresive, characteristic, dynamic. The 5800 is much more laid back, warmer, and also somewhat more natural in it's presentation. Also, less fun, as I mention in the link above. Having listened to iPhones, several Sony-Ericsson models, and quite a few Nokias, I must say it's definitely in the uppermost part of the top league – although not really comparable with a good quality DAP.
I would not recommend it for any kind of serious listening, that's what proper portable players are for (although it does outdo certain iPods, imho). But I do withhold that it is a very nice phone, especially for the money spent, and it's sound quality and music presentation is absolutely enjoyable.
Something to watch out for though, if you're used to handhelds, is that the 5800 is pretty slow to react, compared to the newer HTC models and iPhones. I know some people find it disturbingly slow. Technostress, if you ask me, but nevertheless...