well, we got almost 2" of rain in just a few hours last night so I checked my bed... got just a little water in it on both sides. it looked to be coming in from above the wheel wells. There wasn't enough to really concern me or even enough that it would have got anything wet that I had back there, plus it was an absolute torrential downpour! So, with regular rain I don't think much of anything gets in. I'll take a look when I have a few free minutes to see if there's anything I can tighten up.
I'm still completely satisfied with the Bakflip. The only thing I need now is one of those plastic clips that hold the stabilizer bar along the side of the bed... one broke during shipping.