I use the N900 with a pair of Sennheiser IE8 and I must say the quality is excellent. This week I will get a Fiio E5 and will check if that does anything to the quality.

its good. as good as my iphone, better than my archos/psp. you wont have a problem. unless you are used to sony walkman(newist) quality

its nice although it lacks some features such as equalizer and such

it's funny, but i do not miss an equalizer at all. I understand perfectly , it is a feature some people need. but honestly good headphones are way better than software manipulation.

since i got my n900 my ogg-player has retired.
listening to webradio / last.fm on the go is like old school radio.

i even did not load that much music on my device because i've got all the world to listen to.

I have been using portable music players since the earliest cassette Walkmans, typically 8 hours a day. I was hoping the N900 (32GB plus FLAC) would allow me to retire my Sansa Fuze (16GB plus FLAC). But I'm not convinced for two reasons:
1. size (Fuze is just soooo much smaller)
2. battery life (Fuze will play for 24 hours per charge)

The audio quality of the N900 is very good through my Bose in-ears. I have the complete Beatles Remastered (stereo) in FLAC on both the Fuze and N900. I would compare them as follows:
- Fuze is brigthter with slighty more presence (mid-range) so vocals are somewhat more prominent. NOTE: I'm running with the equalizer normal/flat. But the Fuze therefore can sound a bit strident. Of course I can adjust the EQ on the Fuze, but that wouldn't be fair to the poor N900. :-)
- N900 has more bass and is smoother overall, less edgy but has slightly less output at the same volume setting.

The differences in sound quality are not dramatic. But other difference are important if you want your N900 to be a dedicated music player. I thinks the big one is battery.

Since N900 is my business phone/PDA, I need it charging at night so I can't realistically be listening to music or audio books in bed on the N900. That's why I'll probably continue to use the Fuze. The Fuze is very smal and will run for 3 nights between charges, and has EQ. That's hard to beat with any phone.

Oh yeh...the Fuze also plays Audible.com files and the N900, well, not yet.

I can confirm the above.
Now that I have my N900 I must say it cannot reach the sound
quality of the Cowon U3 MP3 player.
At least not with that MP3 player application.
I am using Sennheiser and Koss Porta Pro headphones.
The N900 sounds somehow "flat".
The MP3s are ripped from my CDs with quite good quality.
At least so good, that sound on the Cowon is very "moving". ;-)

The strange thing is: if I listen to the sound of the videos that came
with the N900 (it's a preview of the film "9"), the sound is much, much better and equal to the Cowon!

I suspect that the MP3 player application is doing some bad
thing to the MP3 files because obviously the N900 is able to
do better sound with the hardware.
Does the videoplayer use some different Audio hardware within
the N900?

Is there a better MP3 player application than the built-in?
Preferable with equalizer and some more possibilities?
One which uses the same Audio hardware/software as
the video player?