I've used the Martin Fields protector, and it seems fine. From what I can tell, the official HTC protectors have a small hole for the green notification flash. Quite a few people end up with air bubbles forming around this hole. I can see the green flash through the Martin Fields protector without a problem.
What might be more important is a hole for the proximity detector. Twice, since putting on the screen protector, I've found that the phone's display doesn't turn back on when I take the phone away from my ear. I don't know whether the official HTC ones have such a hole, but the Martin Fields ones don't.