I've been climbing on the Rope Runner daily for almost a month now and have been testing out a couple unconventional strategies for using it in addition to its intended uses.
It works just as well as a hitch in a DdRT setup, no better no worse. Caveat: I'm used to mechanical device--lifetime hitchclimber will find it twitchy. Its utility for DdRT is a huge asset for me, as I switch between systems a lot. Kevin mentioned that there is a risk of the carabiner attachment on the bridge getting caught on the beak (essentially the lever) of the device, failing it. The risk is pretty small--comparable to the risk of a scaffold knot terminating your climbing system on a hitchclimber pulley failing your prusik--but is easy to mitigate. Either tying off my climb line to the ring on my bridge or extending it with an eye-to-eye sling does the job nicely. I'm sure there's an even more elegant fix, though.
I've gotten pretty comfortable climbing on a RADS with a Grigri 1, but it's no longer on the market, so I decided to test out the RR low-and-slow. It works like a dream, but using it for this application is unadvisable for a funny reason. The hip thrusting motion of RADS ascents brings the top of the Rope Runner directly in contact with the hand ascender, failing the RR. No big deal, as the RR would normally catch again as soon as the climber fell below the ascender. However, a climber suddenly in free fall is likely to do one thing: continue grabbing the rope. I've tested this out a few times, and it's easy to add enough friction to the already efficient system to prevent the camming components of the RR from engaging. This results in free fall with one hand on the rope providing the only friction in the system. So, sadly no RADS for the Rope Runner unless you figure out a way to make one that works upside down.
For its intended purposes, it's great. All in all, it tends slack like a dream, hell you can tend it with your knee as you climb if you need to at all. I really like it.