I think the game is a lot of fun. Is it the GOAT? Nope. I still think MM has the best balance of any game made to date. The callouts alone to me make that game so F'n funny. Shots are great, etc. With that said, when I get random people coming down the stairs that don't play pinball, they jump right to GnR. They play it and love it. They play my other games and go back to GnR. Other games I have are TZ, MM, CV, and WoZ.
My point is it is a fun game for a lot of average pinball folks. They can get a pretty easy multi ball experience. They get to pick a GnR song they like. As for why are so many for sale, well there are thousands of them out there. For some people it wasn't a match. Why is this so hard for people to comprehend? I also shake my head when I read they played one game or even a couple balls and will "never play it again". You need to give yourself time to understand the ruleset. That can take a pretty long time depending on how deep the ruleset goes.
Bottom line is these kinds of threads drum up the bad side of PinSide. Too many cynical posts IMO. We have to realize that there is a game for everyone in the pinball world. Just need to find it. If you don't like a machine, sell it. If you like it, keep it. Peace