The seal is actually pretty good, although I don't know how it compares to other tents. One side is a slide-in seam, and one continuous zipper connects the remaining 3/4 of the cover to the bottom platform.
However, there is not a perfect seal where the zippered ends meet the slide-in seam on two corners. Don't get me wrong - it is a good imperfect seal (you might be able to jam a pencil in the gap), but like Kurt said, when you're cruising along at 65 in the rain, you're bound to push in some water.
My concern is that it was slightly more damp than I expected - a small corner of the mattress and a couple of walls. For all intents and purposes, the rest of the tent was dry and completely usable. It just makes me wonder how other RTT's fare.