Hi there, i am interested opinions on this topic also as i've been considering an s2.... from what I've managed to research about the the galaxy s2 it sounds a bit substandard out the box but there is a mixer program called voodoo that significantly improves the sonic quality by unlocking some extra ability of the s2's sound chip; i've heard the s2 is actually pretty good once voodooed in this way but id love to hear some users opinions.
While i'm here one player i do have good knowledge of is the cowon D3 that DragonBottles mentioned, and yeah on paper it is impressive and truly it does sound fantastic. Ultimately however the lack of processing grunt will likely really piss you off in the long run, sadly mine goes unresponsive frequently when playing flac or anything that uses alot of resources. Personally i would also avoid this player for the reason that the after sales service stinks, it took them about 5/6 months to repair my screen after it cracked on holiday and then charged me about £110. Ive since noticed that there seems to be an awful lot of flexing in this screen compared to any similarly large screen mobile i've ever held.... i wouldn't make the same purchase again, just saying
EDIT... actually ive just read voodoo is only for the galaxy s1 and that the s2 has a worse chip, perhaps this isn't the way to go then