From the
wiki: "Rockbox has a dual-boot feature. To boot into the original firmware, shut down the device as described above. Turn on the Hold switch immediately after turning the player on. Once Rockbox displays the message “Loading original firmware ...” you may turn the Hold switch off. Rockbox will boot into the original firmware."
In terms of usability it's as good, and with some plugins and themes is better, than the default firmware. The only problems are obviously HD space and battery life, which I clock at around 5 hours on my 4th gen colour. The sound quality increase and the ability to use open source sound files outweighs those problems for me. I don't use itunes anymore as rockbox doesn't require it and the music folders can simply be dragged over to the ipod like a hard drive and it will pick them up.