I bought a couple of these for climbing challenges (well, an upcoming Everesting attempt). In addition to the self-guiding cut-outs, the bottles are slightly oval to help the rider orient them towards the attachment points. I'm sure you can get used to this system but I haven't yet: they're not for high-speed use and certainly not for downhills, as far as I'm concerned. When you do get slightly caught out by an upcoming turn, my instinct is always to throw the bottle (although I haven't done it yet). I have also launched a few on rough descents. They're probably slightly better than what you would expect, but they are certainly along the lines of what you would expect.

Incidentally, I always thought of them as ornamental, with climbing use a hack/bodge;-)