FYI I just got one today, after 20 minutes it gets to 2.06 Gh/s with clock=x0981, and it runs great with zero errors and zero rejects.

When I overclock to 0A18, which on the U1 is 2.2 Gh/s, after running for 20 minutes it only gets up to 2.12 Gh/s with 0 rejects and 5.41% error rate.

So, it seems like 2.0 Gh/s is the max you can expect with low error rate.  

I haven't been able to find a user manual on the U2 yet however, and the clock parameters for overclocking may be totally different than what was used for the U1.

I have emailed Antmain about this but have not heard back yet.

Edited to add:

After 2 hours it's hashing at 2.063 Gh/s, with zero rejects and zero H/W errors.

Also, FYI my USB power is not an issue. I am getting my +5V from the molex connector of an ATX PSU at 32Amps.