I have a slightly smaller version of this
http://www.bikemania.biz/Vaude_H2O_W...ude_722190.htm which is very neat -- it clicks on and off the seat post really easily and fits a jacket, some snacks and a few other things -- just enough for a day ride when you're not sure of the weather. It's been stable so far. Right now I am looking for a second clamp so I can move the bag from one bike to another without taking the whole seatpost out.
I'm not sure I would use it with a carbon seatpost though.