There is an excellent comparison here with the X3, C3 and many others...
A Rockboxed Clip+ is a great little player!
The C3 has a much better sound quality and an all Metal construction. It has MANY quirks that take a lot of getting used to, If you only want to play audio from Individual Folders with No Play Lists, Shuffle All, Gapless without using Cue Files, Totally Non functional EQ, 12mW output so it will require an amp for most full size headphones. You have to run your uSD cards through a directory sort program as it will play the songs in the order they written to the uSD card... Have I scared you away yet?
I really do love the C3 as a DAP! I load up my music, select the folder and Enjoy!
If you are a Random Play, Shuffle & Custom List / EQ kind of listener then the C3 is defiantly NOT for you.