When the casings fly out of the bottom facing ejection port they bang up the ejection port cover (a little used plastic piece that you can use to lock open the bolt group), the plastic front of the hammer group, which forms part of the back ejection port, and a little bit of the plastic of the ejection port itself. Simple physics, like the brass marks and wear on an AR-15's brass deflector. I have 3,000 rounds through mine and it is not particularly bad IMHO, and far from wearing out the plastic. Some more obsessive types velcro it to prevent the dings.
I don't suppose I would buy it as my only gun, but I have AR-15s and mutiple pistols. I bought it because of its unique operating features, its looks, because it was something I had never thought would be available on the commercial market and suddenly it was, so as a "toy" (like most of my firearms, in truth). But I do use it as a home defense gun as well, I think it is better than my 9mm pistol in this respect. I did have mine (I just bought a second one) SBR'd and I think it looks a lot better with the short barrel. The original sight, while adequate for it intended purpose and state of the art back in the day, does wash out and I replaced it with a black reticle sight which I like a lot. They are now coming from the factory in a version with the black reticle sight for two or three hundred more, the "USG" model, I might try to get that intead of the standard model.
Should you buy it? I wouldn't if it was your only gun, or it would cause you financial hardship, but I think it is a fun and interesting gun that fills a niche between rifle and pistol. If you can SBR it I would consider that as well.
ETA I find it a lot of fun to shoot as well.