I have an 4g ipod. The sound is definitely different with the RockBox firmware. It seems from the postings that a lot of people like it better. I found that I can hear better tones and more detail with the original firmware though. I cant get the same sound from RockBox, no matter what I do with the sound settings.
I got an old Archos Jukebox 6000 with the intention of putting 100GB hard drive it, and it sounds really great, for some reason! (I dont know about the original firmware, I only tried it with RB). Unfortunately, there aren't enough buttons and the display is so terrible, it is almost impossible to navigate the menus, etc. It is a hassle to just change the volume while playing. Also it doesnt charge over USB, battery life is short, it is bulky etc. Oh well.