I'm generally not a shooter gamer, so I played around with the original Borderlands and it was fun, but for some reason, the mechanics just didn't work for me and ended up putting it down. But when Borderlands 2 arrived, I heard so much about it that I had to give it a try. I still don't like shooters, but this game is just super-fun. Whether it's wielding guns that set your enemies on fire, or hearing one pop culture reference after another as you're making your way through the game, it's just fun.
Co-op play with friends is seamless and easy, except that it's best to play around the same level and same missions as they don't share objectives well (literally they do, but if someone's completed a mission and you haven't, they may not see everything you do).
Highly recommended for entertainment value alone, probably not best for little kids, but frankly you shouldn't consider letting them play shooters at all!