the ar15 has a few thing that are very well designed on it: the bolt, the magazine, the modularity, the in-line barrel/stock is a maybe. It's controls kind of suck though. I know, blasphemy! our beloved ar isn't perfect?!
The safety isn't ambi, and gets in the way of your trigger finger if it is. the charging handle is just a stupid design (if it was a good design is would be copied. is has not.) the A2 grip is uncomfortable, the DI gas system is..... well, it is what it is, and I just don't know how you're supposed to lock back the bolt if you're a right handed shooter unless you are willing to juggle the gun. It works ok for a lefty, but it's a bad design.
The various BAD style levers are as good a fix as any to try and overcome some basic design flaws with the AR, but they are also not an optimum solution. Nobody really likes the idea of extra stuff in the trigger guard, and as trackskippy pointed out, they are easily bumped, but it does mostly work as shown/advertised. Whether or not you need/want the abilities it offers is up to you. Personally, as a left handed shooter, I don't need or run one. But then, I'm not a hard core run and gun shooter/operator either. I hunt pigs and shoot cans.