I have a couple (they are sold in pairs) and for the money they seem OK. I don't think they are intended as durable items to last for many years but they pack up small and are easy to keep handy in your camera bag for those surprise moments. At least they are definitely waterproof - hard to go wrong with a polythene sleeve really.
As for purchase/grip on the hood, I haven't tried it myself but I guess you could stretch a couple of rubber bands around the hood itself and then the sleeve could be tightened around the hood between the bands, thus stopping it slipping off the hood in either direction. Alternatively, or as well, it may work better to secure the hood end with a rubber band around the sleeve, rather than the draw string. Then you would maintain a firm tension that should help reduce slippage.
Here are mine in action (sort of)....
I did see some camo sleeves for £15 (+P&P?) online recently but have lost the link. If I can find the link I'll post back.
Here it is -
http://www.youfreeweb.com/CamoMac/