For those music player gadgets that can run Rockbox, it's the best way to run them. (For most of us, that is a Sandisk e2x0 or an iPod.) The feature set is too long to list; suffice to say that you will continue finding pleasant surprises as long as you continue exploring.
My only complaint is that the theme it installs with is an embarrassment. Anybody who fails to discover one of the alternative themes soon enough may give up in disgust.
Besides being useful directly, the Rockbox project is an excellent introduction to embedded system programming. If you would like to break into that field, but nobody will hire you without experience, here's your chance.
I discovered Rockbox back in October of 2006, but I had no supported DAP to try it on. When I received a 30GB iPod video for Xmas, I went straight to the Rockbox site, downloaded it, and installed.
And I was floored.
Rockbox does everything Apple should have done right from the start: true drag-and-drop support (via Apple's Disk Mode or OF), playback of many different audio codecs (including favorites Ogg Vorbis, FLAC, Musepack, and others), plugins, better battery life than the original firmware on many of the non PortalPlayer-based targets, and growing improvements every day.
With a growing list of supported targets, an excellent, very mature code base, and more features than you can ever imagine, Rockbox deserves its place on any savvy user's DAP!