I have a duo and a quad and may have to plunk down for another quad soon. (They have historically had a big sale in November or December.) The UAD plugs are some of my favorites and I have been using them more and more. With the last upgrade that gave full RTAS versions they work much better with Pro Tools. You should talk to someone running them with Sequoia/64 bit. They are still at 32 and I hear the 32/64 bridge. Since Pro Tools and UAD are both at 32 at this point I can't report on that.
Favorite Plugins: The compressor collection in general is great. I don't have all of them yet, but I love the LA2A. The 1176 and Fairchild are next. All dead simple to operate of course - like the analog units they are trying to emulate. I assume they are not exactly like their hardware ancestors, but I have one of them on every track and bus. It's not like I can ever dream of affording an analog collection like that. I don't have the Fatso or the dbx 160.
The Manley massive is the best EQ plug I have ever used. DSP hog so you can't use on on every track, but it's great.
[corretions added] The reverbs are maybe the highlight of the unit. (Of course, they are cpu hogs, so pushing that function to a DSP card can be a good idea. One way to think of the UAD is as a cheap substitute for a Bricasti.) The Realverb Pro is their [free with the card] "natural" or modern reverb. [They have a $149 upgraded unit called the "Dreamverb.] I have not played with it the Realverb much. When I first tried it out it seemed to have the same aims as the Sonnox Reverb that I own, and since I already knew how to get what I wanted out of the Sonnox I didn't explore further. The two premium reverbs that I own are the EMT 140 plate and the Lexicon 244. The 140 is the best plate plugin that I have heard. Sounds very real to me. The Lexicon is a very good emulation - instant '80's. I've been setting up three Reverb buses on most songs: a small room (Sonnox), a plate (140), and a wash (Lexicon). I use them in varying combinations on all sources.
I also have the Studer Tape emulation. I has a very nice effect on all tracks and really provides a unifying sound. Problem is that when you use in on all tracks you use up a quad card pretty quickly and don't have enough DSP for using all of the other plugs. They have a new Ampex emulation which is supposed to sound better across the two bus. (The Studer is OK across the two bus, but sounds better on individual tracks in my tests.)
Of course when you get the card you get all of the plugs to try as 14 day demos, and they renew the demos each time you buy a new plug. It's like handing out free samples of crack. The plugins are that good.