Let me say, first off the device itself is great, and a real game changer for road riding. No question this will save lives!
That said, the o-ring and mounts are garbage. It is always mind boggling to me that companies will put all this great technology into something and then go cheap on a crucial part of it. I've lost my share of devices off the back of my bikes on bumpy roads, weather, etc., so I know they really need to be secured very well back there. This one is not secure at all. How would you like to shell out $200 bucks and lose the light on the road?
The problem is that there's a very dicey bracket to hold the o-rings (how hard would it have been to make it secure the o-rings better?). And worse, the o-rings themselves are too small to fit around the seatposts of higher end bikes. The "large" will not fit around the thick seat posts that you find on aero bikes, TT bikes etc. You have to rig it - while that's par for the course with Garmin, I find it obnoxious. If you have thick seat posts, I would not buy it until they provide better mounting. Will they? Who knows? It's Garmin. They don't have enough competition to care!
*****Update. Just as I suspected, even with a rigged setup, one of the rubber backing pieces came off on the ride, and my Varia dangled off the back for miles. Could have easily lost the whole thing. If you have a bike with a wide seatpost, do not buy this product