They have been around for a long time. They are a branch of CD Universe. I do seem to recall CD Universe having some kind of problems around 3 years ago but haven't heard of any complaints since then.
There are a lot of folks using Adult Video Universe through the price search so hopefully you'll be able to get some real customer feedback. (I haven't placed an order with them.)
I don't know anything about AV Universe but I have purchased quite a few CDs from CD Universe over the last year and have had no problems with them whatsoever
I've ordered from them 5 or 6 times and I've been very happy. On one order I had a problem with a VCX DVD that didn't play. I sent it back to them and they replaced it with no problems and credited the shipping amount to my account. So I can attest that they handle any problems fast and efficiently.
I've used them off and on for titles that other stores don't seem to carry. I've never had any problem that they didn't resolve to my satisfaction (I received a bad disc once but had no problems exchanging it for another). I'll continue in the future to use them off and on as well.