I was a bit scepticle when I first saw this bottle, but thought I'd give it a try. Actually really like the system.
The cage is basically two plastic knobs that weight nothing. The bottle connects to the knobs. You get four knobs with a bottle, so basically two cages... I bought two bottles and have four cages to put on my two bikes. SWEET.
I've been riding with the bottles on some gnarly, rocky MTB trails and have yet had my bottles pop out. Also use them on my road bike with no issues losing bottles.
It takes a few times to get used to taking on and off the bottles, but then you get used to it.
I'd say this isn't a solution for road racers, however I don't race anymore -- just ride my bike all the time. It'd actually be a cool option for a TT bike and/or cross bike where you want a bottle for training but not racing.
Overall, great hydration system, which really cleans how the bike looks with water bottles on.