Saw it run at the 12hours of Sebring. It's VERY competitive, but the Pugeots are just as good. One thing they did say was that the drivers love the R15. It feels nimble like the R8 (not the production car) rather than heavy like the R10.
However, if Acura can iron out the bugs in their new P1 cars, they could be a real force to be reckoned with. Their cornering grip should be incredible.