I've had the Quadra for nearly a year now, and I would say that I'm mostly pleased with it. It does a good job of ridding my living room of odors (from cooking or whatnot), and if all of the gunk that I wipe off of the collector plates is any indication, it does a dandy job at cleaning the air too. The lack of a fan and replacing filters makes the Quadra a much cheaper unit to operate than your typical HEPA filter (and MUCH quiter...usually -- more on that later). The faint ozone smell that it gives off is pretty pleasant too; it reminds me of the smell of the air after it has just rained.
My one complaint is that it will sometimes start making these annoying static-y noises if a little ball of lint or something gets in there and creates a little "hot spot". Oh, and it made similar noises once that didn't seem to improve after cleaning the collector plates. Upon close inspection I found that there was a buildup on the two wires that run vertically in the Quadra. I ended up rigging an old toothbrush on a long dowel to clean those off. The owner's manual doesn't mention anything about these wires needing cleaning, and they sure as hell didn't design them for easy access. That hasn't been a problem since then, though, and really wasn't a major hassle to take care of.
Bottom line, I would buy the Quadra again if I had it to do over. If you decide to get one, be sure to check out eBay. I got a refurb unit there for well under $200. Oh, and I would suggest getting the darker colored one since it probably won't show the sooty stuff that tends to build up on the plastic of my light grey unit over time.