When I used Rockbox, it didn't have any features that my iPod didn't have
Besides a lack of need for iTunes, more codecs (ogg, flac), skinning, better selection of games (e.g. gameboy emulator, Doom), voice recording... totally worth the battery life dropping by a bit.
Regardless of where you stand on the V0/FLAC argument, the iPod can't reproduce the quality of a V0 (to be precise, this is only true of the headphone port; the line out on the dock connector should be near-perfect, but Rockbox is incompatible with anything that uses that), so there's no point in putting a FLAC on an iPod even if you do have Rockbox installed. The iPod can already play Apple's lossless codec, and iTunes will convert incompatible files automatically for you.
Rockbox does not work with my car stereo (pioneer), which uses the line out on the dock connector.
I've never had any trouble getting around using iTunes, between Winamp/Songbird for Windows, or Amarok for Linux. Also, I don't understand why people hate iTunes so much; it used to be unstable in Windows (emphasis on used to be), but there has never been anything inherently wrong with it.
I don't really care about skinning on a portable music player, it's going to spend most of its life in my pocket or glovebox. I actually found the Rockbox user interface pretty hard to navigate, which was a huge turnoff for me.