As it happens, ViEmu has just launched their product on XCode as well. From the couple of days in use, I'm quite pleased with it in comparison with Vicious that I used before that.
xvim is super alpha... the visual studio guys just released a vim plugin for xcode too... I haven't used the xcode one yet but if it's as good as the visual studio one it should be very good. (I have contributed to xvim and I have purchased the visual studio plugin)