Welcome to Mistic River.
Is gapless support important to you? How about a parametric equilizer? How about OTF playlists? This is just a short list of features that Rockbox supports. I suggest to look in the Wiki manual to look at its complete set. Here is the link:
http://www.rockbox.org/twiki/bin/vie...Menu#Crossfeed
And by the way: You can always boot back into iRiver firmware if you need to after Rockbox is installed.