I've had each generation of iPhone...and it seems for each, I've ended up with two or three cases each...finally, with the 3GS and on to the 4, I was happy with the iFrog cases. Decent protection, sleek, tight fitting and all Ports accessible. Usually at Best Buy, they'll go on sale 50% off once in awhile
That said, with the purchase of my new 4s, I picked up the Mophie juice pack Air. Love it! Adds about a third of an inch on the bottom...no significant weight considering you're getting essentially another battery! It's awesome...and again, tight...sleek, port accessible and looks to be protective in the case of a drop
Highly recommended. However, there's a model a step above mine. They were out when I got mine, otherwise I'd have done that. A bit more power storage. Regardless, I wasn't one with battery issues either with my iPhone 4 (with iOS 5.0) or my 4s. Both were fine. The Mophie though adds an extra sense of confidence. I can tether to my MBP and MBAir all morning. Turn the juice on....and go from 40% to a full battery within the hour! Splendid