I have the Magpul Field Case on my iPhone and I love it. It adds very little to the overall size of the phone, but also adds some nice grip when holding it. It's a solid piece of TPU with just enough flex to get the iPhone into and out of the case with minimal hassle, but not a floppy, stretchy mess like a silicone case. The buttons are covered up nicely, but function just fine. The power button takes a little bit to break in, but, the volume buttons are spot on. I picked one up in orange and flat dark earth, and have been strongly considering buying a black or OD green one to add to the mix. The quality on the case is top notch, and the price really cannot be beat. If you do a little shopping around, you can find them shipped to you for just a shade over 10 bucks. I have yet to find a case of this quality for even twice that price. I highly recommend it.
Edited to add: depending on your dock, you may have to remove the case. It's a slim fit, but on the side of each end of the 30 pin connector slot are raised ribs that may interfere with a flush fit. I've seen some docks that have an extended connector piece that would probably eliminate that problem, but it's something to be aware of.