Monster paintballs, the paintballs that are both loved and hated by players. A lot of players bring them to a BYOP (Bring Your Own Paint) paintball fields, and think its funny when players get hit by them. These are the most painful paintballs to get hit with on the market; it absolutely kills when you get hit by one. I think it takes the fun out of the game a little bit when you get hit by a rock hard paintball. It also makes beginners change their mind about paintball. Last weekend I was playing at a BYOP field, and a kid got kicked off the field because he was using Monster paintballs and the field didn't allow them. Most commercial paintball fields, are not BYOP fields, for that very problem. Many/most paintball parks require you buy your paint from them; it's one of their best revenue sources; plus this way fields know that all players are using the same paint and can ensure the games played there are fair. I'm happy with using whatever paint I can get, but if it means getting monster paintballs, I won't buy them. I love playing paintball, and it really ruins a game if someone's using paintballs that are illegal at most fields. Its best to just buy whatever kind of paint the field has, this way you have a fun safe day without anyone getting hurt. I recommend not buying Monster paintballs to all players. I would recommend a very good, reliable brand of paintballs called Karnage (sold on this website). Karnage paintballs have a great shell formula that's hard enough to be reliable in your barrel but soft enough so it doesn't hurt to bad (nothing like Monsters) when you get hit. They also have several different grades and I've found even their lowest/cheapest grade - Karnage Bite Recreational Grade Paintballs are totally reliable and better than most company's top tournament grade. 
Mike Morton, Team Trinity