I guess I'm just incredibly lucky. I have not any leaking problems even living in sogging Seattle, going through high pressure washes, heavy snow, etc. etc. I do have water under the mat (dampness would be a better word). I also have had dribbles when I open the tailgate, but that is because the seal between panel three and the tailgate is broken when you open it and the water comes down. I've seen evidence of water in the gutters, but never seen water in them. I've seen no evidence of water "flow" into the drains or say when I'm parked (small puddles in odd places). I've had no water go between the length of the panels - 1, 2 or 3 (except on the forementioned tailgate opening). I've seen no evidence of water coming through at the gutters and down the side. Also no water through the midgate - that includes when I was an idiot and closed my rear seatbelt into the midgate and had one piece of the middle bar unlatched.
We've had some serious, serious rain and snow this winter. Stuff on the mat should stay dry - period. Water coming through at the gutter is possible the system should keep it away. Water coming through the long seams on the panels is not acceptable - period, and a dealer telling you that it is is selling you a line of bull.
I tell you what I've told other people. Would I carry the Mona Lisa in a pouring rainstorm in the back from Point A to Point B? Yes - no questions about it. Would I keep six 100 pound bags of concrete back there for three months? No way in hell.
Just my two-cents...