The AP-181 fan only has a ~200RPM faster motor. The key difference is the way
it pushes the air, and it is pretty incredible.
(This is the 120mm verison, but it has the same design/concept as the AP-181)http://www.youtube.com/watch?v=8m8fC809TK0
The only drawback I've noticed is multiple GPU setups with external exhaust cooler (most reference coolers), since the air does not want to flow sideways into the GPU cooler (it doesn't overheat, just runs a bit warmer).
I had 2x470 with the regular fans, and I think the AP-181 would have made a difference.
Note that the RV02 does not come with the 181s, but I believe the RV02-E does. I don't think the -E version cools as well though.