I got one, and love it. Wait, there are two... never mind, I've got three if I counted right.
On on a shotgun, one on the PS90, and the third sits on an AK. They all get used regularly and none have failed yet.
All mine were purchased before the new ones with the included riser, so I can't really speak to how useful it is, but a test fitting at the LGS gave me the impression the Burris riser was just a bit too tall. I'd go with a UTG for such a low impact item.
If I remember correctly--
High mount (25mm) for lower 1/3 co-witness
Med mount (21mm) for absolute co-witness
Low mount (13mm) for ...mmm... other stuff.