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.
This library was converted from the successful Xerces Java library. It is enormous and leaky. Of 18K+ Debian packages, only 3 actual programs use it. You would be well-advised to use libxml2 instead.