I have the Marware Flip cover on my iPhone. I really like it. I had gotten so use to the flip cover that I had on my iPod Touch, that this was the first case I tried. Plus I wanted a case with a clip, but didn't want to have to take the phone out of the case to use it.
The only thing I wasn't sure about was how it would work with the flip cover when talking on the phone, but it is no problem. Really good case in my opinion.