You attach the case by snapping it onto the back of the phone (how else do you think it got its name?) Removing it is just as easy; just pull gently on one side and it pops off. When I was using the case I had no problems accessing the controls or ports and it didn't interfere with taking photos.
Without a phone attached I subjected the Impact Snap to a fair amount of abuse. I hit it with a hammer, stomped on it with a heavy shoe, and scratched the D30 section with a knife. Each test left no marks, though take note that you will leave a scratch if you take a knife to the rubbery material on the case's rear side. I didn't think that was a concern, though, since the Impact Snap is saving the back of your phone from the same fate. Squeezing the case in my hand caused no damage either. As soon as I released my grip it snapped back into its natural shape.
While it was wearing the Impact Snap I threw a Galaxy S3 on the floor several times, banged it against a counter (rear first, of course), and smacked it with the handle of a screwdriver. Each time the case did its job of absorbing the blows for the handset.
I wish, however, that the front of the Impact Snap curved farther around the front of the device. Though the end of the case is slightly raised above the Galaxy S3's bezel, I doubt it would be enough to protect your handset if it fell face first on an uneven surface (a flat surface shouldn't pose a problem). I was unable to prove my theory, but it made me nervous just the same. Granted, expanding the case would make it bulkier, but I'd want to protect that big, beautiful display.
As this is the first Galaxy S3 case that I've tested, I'm unable to say that the Tech21 Impact Snap is the best on the market. Even so, I'd recommend it strongly. It should protect your Galaxy S3 from serious abuse while letting you show off the phone's eye-catching design and luscious screen. Sure, I wish it had a little more protection around front, but it should absorb the blows of day-to-day use.