I'm an iRiver user, so I don't really know the differences between what rockbox offers on the iPod vs the iRiver, but here are some of the enhancements that made me go rockbox and never go back:
1) Gameboy emulator. Simply download a rom and execute it on your unit, and voila, instant gameboy game.
2) As was stated before, instantaneous bootup, as well as loads of other little features like changing the font used, etc.
and the main one that attracted me to rockbox:
3) Enhanced recording support. On the iRiver firmware, for some reason there was a 750mb filesize limit on the files you'd create while recording with it. So, when recording to wav it would only record for 75 minutes at a time (at which time you'd have to start recording again which would take about 10 seconds to actually start). Now, I'm an avid show taper, and have completely switched from DAT to the iRiver for recording shows. Rockbox completely does away with this stupid limitation, and increases it up to the fat32 filesize limit of 2gb. So, now I can record for over 3 hours straight. Also, recently peakmeters were added (and in the iRiver firmware you can't even control the input level when using line in, only for mic in). There's also pre-recording, which uses a circular buffer (of a length you specify) when you open the recording screen. So, I set it to 30 seconds, and once the band gets on stage I hit record, and it includes the last 30 seconds, so I don't have to guess when to start recording. Now, I don't know if there's even recording support for the ipod, although I've heard it might be possible through the headphone jack? If so, it would definitely be worth it to switch to Rockbox.