i have an old ipod 4g photo - 30gb, that has taken the backseat to the 8gb ipod touch i got free after rebate with the EDU discount for my macbook pro. i decided to rockbox it (the older one), and so far i think it's a great idea! you don't have to give up the regular ipod firmware, instead it's like having a "dual boot" machine. you can still boot up into the normal ipod software by flipping the hold switch to on after you power it up, otherwise it will boot into rockbox by default.
i'm now de-selecting "sync music" in itunes to have it take all my old mp3s off it (for the time being) and will load it up with some flacs and see how it does.