I use my dad's old PDA for music...he didn't need it after his company switched to BlackBerry phones, so I took it (a Dell Axim x50v) and installed some music player software on it.
The thing's so good I don't think I ever want to change! It doesn't have the size of other players (just a CF and SD slot) and it isn't as good for organization, but it has tons of enhancement programs to make your audio sound great through headphones or speakers. I have GSPlayer which has tons of options, SRS WOW HD with TruBass, and X50mix [a simple EQ that controls the hardware equalizer of the x50 series]. It also plays videos and YouTube and stuff...it's like a mini computer.
However, it has horrible battery life, the 624MHz CPU eats up the battery very fast. I made an external battery using an AA holder and a 5v regulator, lasts about 6 hours on that using 8 NiMH batteries...enough for a movie or two and some music.
My friend had an iPod 80GB but he dropped it on the floor. It didn't work after that because the hard drive was ruined. He also had an RCA hard drive player before that...dropped it on the ground too and its MicroDrive broke...I think Flash technology is better for portable applications because it can survive (I threw an SD card up in the air and let it hit the floor, then threw it at the wall, etc...and it still works just fine, but drop a portable hard drive 3 feet onto somewhat hard floor, and it's ruined).