I tend to just use them in groups of 6 with either 2cluster caltrops or two heat lances. The Heat lance unit won't bring caltrops cause they will eventually slow down enough to lose their invul save to blow up a tank (again, there are better ways to blow up tanks in our list, but I'll stay on the Reaver topic) and they are just too pricey at that point to be a unit that more often then not will come under return fire the minute something big gets blown up by them. I also bring (more often then not) a unit of 6, or 9 if you're feeling randy, with JUST cluster caltrops. Again, points are at a premium in most games so I try to specialize. This unit goes a turbo boosting around the board the entire game bladevaning stuff and getting 3+ invul.
Granted there is way to much flamer nonsense in 5th to give them much hope of surviving long. I've noticed they take an odd amount of fire from my opponents most games so that's one of the reasons I bring them, but again, there is no reason to dump too many points in them. They WILL come under fire, and they WILL die. Or they will get assaulted and well its pretty much a wrap. But, if they can draw even ONE turn of enemy fire from the REAL threats in my army then I think they've done their job at 172pts.
I think of them as a harass unit. Nothing more. If you get lucky, you can wipe out a Tau scout squad or blow up that Land Raider hanging out trying to flank your Wracks holding that objective. I give these guys a 84% cool factor for models alone, and you will see, because of that cool opponents tend to pay them a bit more attention then their worth most games. Jus' sayin'.