Here's my summary after trying five cases (on two phones):
I prefer the Evocel Dual Layer (mentioned by Guytronic at the beginning of this thread) with the Boxwave a close second. I also tried the Amzer Pudding, which was pretty good but a little loose, then came the poor-fitting Lightning Knight. The OEM MS Flip Cover is a different style case, but is really nice if you need that type.
I got all from Amazon here in the US, except the flip cover came directly from MS.
On one phone we use the Boxwave I mentioned in an earlier post. It's a tight fairly soft single layer that gives good protection but apparently is available in only one color- smoke gray.
My favorite is the Evocel. It gives the best protection, seems well made, feels good in the hand, and I like the positive 'click' in the side buttons. A real plus is it has the best front lip or bezel to protect the screen. Another plus is the wide color choice. My headphone jack fits fine, contrary to other reports.
If you're looking for a slim case, though, this isn't it. The sides are somewhat squared off, so it adds some bulk to the phone. If that matters to you (I actually like it), look elsewhere.