Based on this thread I ordered a set of G-Form knee pads last week, which arrived on Friday. My only comparison is with my Endura MT500s as that's the only other set I've used. The MT500s are generally considered to be on the more comfortable end. More a trail than a DH pad. I can tolerate them on 2-3 hour rides, but am always happy to take them off as soon as I can.
Taking the package with the G-forms out of the letter box it was already clear that these were going to be a different proposition. They are much much slimmer than the MT500s. Slipping them on they just felt like a pair of lycra knee warmers with a little bit extra on the front. I tried a quick drop to my knees on a tiled floor though and they did indeed seem to work.
I used them for a 3 hour ride yesterday and hardly noticed them at all. The upper gripper slipped down a bit after an hour or so, but still wasn't uncomfortable and it was easy enough to pull them up again. It will be interesting to see how they perform as they wear/stretch though.
I can't really comment on their protection as I didn't fall off. Never seem to when I'm wearing pads Although the coverage seems OK my guess is that they wouldn't give quite as much protection as the MT500s. However, they certainly provide more protection than the MT500s do when they are sitting in the cupboard at home, which is where they tend to stay. The G-forms are so comfortable that I can see myself just sticking them on for every ride without thinking about it.