I ran the full marathon.
If I was going to run the RnR in Vegas again, I'd opt for the half or the 10k. A full marathon in Vegas is just too boring, flat, and uninspiring. It's one long dark stretch of road after another, with no end in sight. Security is sparse, as are nutrition/water stations. Early water stations are very well organized and packed with volunteers. Mid-race stations around mile 16-18 (when you really need them) have very few volunteers and are uninspiring.
As far as spectators go, well, this is Vegas. Expect lots of drunk people cheering you on. Running through Fremont St. at the beginning of the race was a blast, however the return trip on mile 22 or so was horrible. Nothing worse when I'm hitting the wall then running through a belligerent crowd of people in a tunnel full of smoke.