I haz one.
The only time I ever used it extensively was when I did the pistol course at Tigerswan, and to put it mildly it was not all I hoped it would be.
Me, I mean. The holster was fine, I just sucked drawing from it.
My experience caused me to shelve the holster for everyday carry (not that it was a big deal since I usually carry IWB anyway and the SERPA is more of an open carry rig) although I will still wear it to the occasional open carry events that I attend. I wouldn't depend on it for an everyday rig, but that is strictly because I have a lack of confidence in my ability with it, not that I have a lack of confidence in the rig itself.
My opinion is not shared by everyone, of course. There are a lot of stories floating about the interwebs about how horrible the SERPA holsters are, and some institutions have banned their use all together. There is also a famous video where a SERPA user shot himself that is used to bolster the "SERPA SUX" allegation, but the shooter himself said that it was not the fault of the SERPA holster.
On the other hand, there are positive reviews as well. I think it was more a lack of training that did him in rather than the holster, I don't have a link but I remember seeing that it was not a normal carry rig for him. The bottom line though is that once the confidence in a carry rig is upset it can never be fully restored.
For this reason there is interest in getting away from the SERPA. There are several alternatives out there, everything from positive retention to the tried-and-true thumbstrap. Range Partner has a review of the Safariland over at his place that might interest anyone looking for an alternative.