My experience running Daily builds of NB6 w/ Ruby have been interesting over the last two weeks. The editor and code formatting are great. The rest of the IDE pales in comparison to Eclipse Runtime w/ RDT, RadRails, Subclipse. I like the editor though.
You can switch it off of JRuby on to your regular Ruby & Rails install in the prefs.
IRB in NB is just IRB [ AFAIK ] and not the script/console you get with rails.
The other thing that SUX is that it fires up webrick and there is no way to stop/start webrick (I use mongrel). Even if you quit NB it leaves WB hanging as an orphaned thread a lot of the time.