This is a good product, but as others have noted you have to be really careful with fitting it. It comes as a case and mount in separate boxes. When attaching the mount to the case you have to really push the two pieces together. The mount has a quick release plate (which seems to be very secure in use): take the quick release plate off to fit it to the case and you'll be better able to see what you need to do - push hard until the notches fully engage with a solid click.
I was lucky - I hadn't done this properly on my first ride but it stayed put anyway, although I had fitted the safety cord as a backup (highly recommended, at least until you're sure its all secure). Unfortunately my friend who has the same case also hadn't pushed that plate all the way home, didn't have the safety cord attached, and the case duly flew off on the first ride (albeit about 50 miles in). Disappointingly, as for a previous reviewer, the case immediately opened and spilled the precious iPhone onto the road - smashing the glass on the back. I would have hoped that the case would stay shut, but there you go.
Having said all that, its easy to use the phone through the case, and it seems to keep the rain out. With all the unnecessary toys turned off (WiFi, Bluetooth etc) and turning the screen off whenever I wasn't looking at it, my new iPhone 4S gave me 3.5 hours of navigation using alternately Bike Hub and MotionX GPS, so this is really useful if that's plenty of time, but beyond that you'll need external power. Also, don't forget to lock the screen rotation, as the accelerometer gets very confused in motion.