I have the otterbox on the my 3GS and it certainly does its job as advertised.
My only beefs with it are:
The screen protector started doing the "oiling" thing on the screen almost immediately.....I just removed the film from the case.
My fat fingers are unable to turn the ringer back on from the silent setting which is pretty annoying......I end up using a pen or a fork or something.
The raised ridge around the screen that protects the screen can make it difficult to manipulate things and hit letters when typing near the very edges of the screen......I just live with it.
The bulkiness of the case makes it difficult to use it with other accessories. I have an aftermarket 12V charger that I had to shave down on a sander to get it to fit, I have yet to find an armband for running that will work with the phone in the case, many iphone specific cradles simply will not work with the otterbox installed.
Personally i think otterbox is missing the boat by not making more accessories that work WITH their case.
But honestly, I've dropped my phone a half dozen times and it doesn't have a scratch on it so its doing its job.