Maybe its a bit early for general feedback, so I'll add my view. I've ridden the 66, 71, 76 and 81 quad as well as EVOs and ETs.
The first thing about the quads is that they feel more like a "normal" (i e single fin) board in general riding than a twin. And in fact you have more upwind and planing power than most single fins. The SB twins go upwind well too, but like most twin require some more feel. The Quads are just plain solid fin power. But on the wave the quads are still almost as loose and natural as the twins and looser than EVOs. Similar to twin fin boards, they are really easy to set the rail on and to get a lot of rail in the water also without adding so much power. In a steep top turn they are looser and easier than an EVO but not as super easy as most twin fin boards. But on the other hand they have much more grip than any other board I've used so you can really push it in the top turns and averaging over a variety of waves and top turns I wouls say the Quads are both more forgiving and higher performing (which is in most cases the same thing in windsurfing). You have the grip when you push it, but can also adjust the board mid-turn (like with a twin) when you want. And an extra plus is that is is easier to keep the turn going roundhouse style without loosing speed. And this was in fact the first thing I noticed about them in the bottom turns too. They keep speed (and accelerate) in the bottom turn and you can use that drive both when tightening the turn up to go vert and when drawing the turn out. Now, after riding the quads for a few weeks I don't think about this anymore, but it's stil incorporated in my sailing I think (when I got off the water today people asked me how I could generate so much speed in the short mushy waves, so I guess its noticeable too).
So overall I think the reason for SB to replace both ET and EVO with the Quads is that they combine very efective straight line sailing (upwind planing etc) with super nice and easy _and_ radical wave sailing (with some special quad-feel as an added bonus).
But the best thing would of course be to try them. If you come from EVOs you will feel at home but still be blown away, I think.