I have the Seidio case ($23 from Amazon, free shipping w/Amazon Prime). I recommend it over the Radio Shack case because it doesn't obstruct the capacitive faux-hardkeys; they're annoying enough to deal with as it is without making them harder to touch).
My biggest complaint has to do with the relative placement of the power button, the volume rocker bar, and the case cutouts. When I grab the phone with my left hand, there's nowhere solid to put my thumb besides the case around the screen due to the need for a void around the volume rocker bar. However, at that point, 100% of my holding force is on the screen, with nothing solid to hold the lower half. When I press the power button with my index finger, the force pushes the keyboard open (or at least makes the phone feel disturbingly "jiggly". If I hold the phone with my thumb low enough to apply force to both the screen and bottom half of the phone, my index finger can't reach the power button.
I think most of the problem is due to the gap Seidio left for the wrist strap that nobody (I've seen) ever uses. I think it would be a lot better if Seidio just covered the wrist-strap hole with case, and punched a small-diameter hole in the case above the wrist-strap hole instead. Then, the thumb would have something solid on the lower half of the case to hold on to when pressing the power button with the finger.