There do seem to be availability problems. Even with the very limited number of access points I've dealt with there have been times when I've had to wait a week+ for delivery, or had to purchase a different model from the range.
Once you actually get your hands on them they're fine, and when the V3 software comes out there'll be some nice additional functionality available.
From a cost point-of-view there isn't really a direct alternative to what Ubiquiti offer. If there is I'd like to know.