The first one I had the flap wouldn't stay put. The switch died on it about 2 weeks after I bought it and I had to return it. I doubt that the 2 issues were related.
The new light's flap stays put a bit better and I haven't noticed any water in it. I also have it mounted on the underside/front of my stem so it doesn't get horribly wet. I'm in portland and commute 4-5 days a week by bike.