I've thought about it, but for trad I usually just clip both ropes to the anchor so there's not much need for another backup. I do have a long sling girth hitched to my belay loop, but for the most part I just use the ropes.
When climbing sport, I've thought it might be useful, but clipping the rope and a nylon sling as a backup work just fine. The 48" sling can easily be doubled or tripled to give you three easy lengths; if you just have to be a few more inches one way or another, the PAS would give you a bit more range. But I don't think that flexibility is really worth $20 more than a nylon sling, especially when it's so bulky.
I've seen other pas-like devices at Rumney that were various colors, so they probably weren't Metolius but I didn't get a chance to ask who made them.
I thought I saw a new Metolius PAS that uses thinner webbing two weeks ago at EMS, but I was in a rush so I didn't get to check it out. I'm overseas on business so I can't confirm it. Not too much thinner, just enough to notice it looked a bit different.
If Mammut came out with one using the 8mm floss (or the new 6mm stuff) I'd be very tempted to buy it. But even with the new design I think the Metolius PAS is too bulky.