Hey,
I haven't used it. I've looked it over a couple of times, but I'm more inclined to post here or on RGP for a couple of reasons.
One, it gives me a lot more exposure. When I checked, there were 133 people online here right now, on a Friday morning. Make that a Sunday afternoon or evening, I would guess it would tap 200, easy. I'm guessing RGP is easily in the same range. More exposure means I'm more likely to get what I want, whether that's a new game in my house or a sale of something I want gone.
Second, if I were to venture a guess, I would say that a REALLY good number of people using Pynball found out about it from one of three places: Pinside, RGP, or maybe KLOV, but don't hold me to the KLOV thing, as I'm not there a ton. So, if I post an ad here and on RGP, I've captured almost the entire audience I'm going to get from posting on Pynball, and the only people I miss are the handful of stragglers who Google "pinball classifieds" and happen to look way down at the bottom of the first page of search results. I would guess a lot doing a Google search would just go to Mr. Pinball, as it shows up first. Whether it's better or not is a matter of opinion.
Third, posting here gives me a filter. If some guy contacts me on Pynball, I have NO idea who it is. Here, if I get a message saying, "LOLZ I saw u hd a Whitwatr fr sale rite me back pics mkay!", I can look around on the forums and say, "Yep, this guy consistently presents himself as an idiot, says offensive stuff, and I don't really think he'll be able to scrape together the money for this game, so I'm not going to do this deal." Then I can ignore him. On Pynball, I don't have that built-in background check.
I appreciate what it's trying to accomplish, but I'm of the opinion it's a bit of a solution looking for a problem. When I wanted a Black Knight, I posted on RGP saying what I wanted, and had one two weeks later. Same thing with TSPP; two weeks from the time I was doing a price check on Pinside to the time of purchase, and that includes 1500 miles of driving. If you've got cash in your pocket or a game to sell and you're already on this site, I don't much see the need to go further. Just my two cents.
Luke