The tablet absolutely does NOT interfere with the usage of the grab handle. My wife and I had concern about that initially, but I'm happy to report that she had no issues whatsoever. Regarding the case being "waterproof"; no, I wouldn't call it waterproof, but I would say that it seems to be pretty protective, with a cover over the screen (does not seem to inhibit touch-control at all). As you can tell from my photos, we run a flip-up windshield, so would have it shut during inclement weather. I do feel like it affords an adequate measure of protection for our use. I don't think casual splashes/spray would be an issue, but, if I saw it getting drenched I'd immediately look for a temporary solution to get it out of harms way.
My only real concern is how long the holder will stand up. I'm not concerned about the RAM mount, as their stuff is built pretty bulletproof. The iBolt TabDock Mount seems less robust, but so far has worked well. Notice that I've got a piece of closed cell foam on one of my switch covers on the dash plate, which the corner of the tablet case rides against. This protects the case (and switch cover) and steadies the whole affair somewhat.
MOST IMPORTANT (at least for me): Mounting the tablet high up on the roll cage was my first thought. But, upon positioning it there, I found that it was difficult for me to read, as it's so close that I need to be into my bifocals. And bifocals are, of course, on the bottom of your glasses lenses, thereby requiring you to crane your neck upward in order to bring them into your line of sight for an elevated tablet position. Mounting the tablet where I've shown eliminates this issue altogether and allows me to [somewhat] keep my eyes on the road too. This, of course, may not be an issue if you don't have 64-year old eyes.
Steve