I've been using a GUB for about 2 months. It took more than a month to receive mine from allez-bid.

The claimed weight is pretty accurate, saving a huge amount over the Biologic pump seatpost (around 400 g). The weight savings is of course a bit less when I factor in the pump that I now have to carry (~100 g).

The clamp design is pretty awful. I think it's all geared toward easy manufacturing and lightweight, casual riders. As you've noted, the clamping surfaces are minimal. The lower surface (the edge of the half-cylinder) is approximately 1/8" thick. The upper surfaces are thicker, but since they're at an angle, there's still not a whole lot of material in contact with the rails. The biggest problem, however, is that an imbalance in clamping force between the front and back bolts causes the entire assembly to shift forward. The red arrow points to my biggest concern: the bolt head grinds against the seatpost just above the hole for the bolt carrier. I think this is where the seatpost will eventually break. I'll let you know when it finally happens...

You can also see that the bolts are curved from the misaligned clamping surfaces. Tightening them makes the problem worse.

Other than that, the seatpost is pretty flexy because the tubing is quite thin at 1.5 mm. I weigh <140 pounds so I don't normally notice flex in other seat posts.