I use one and like it. So much I bought a second one.
Some folks say "THEY"RE BAD !!!! YOU"RE GONNA DIE!!!!"
A. Yeah. They're right. I'm gonna die. That's for sure. Bet on it.
B. Nobody's able to explain why they think they're bad. Buoyancy on ascent? I almost always never have any air in my BC at any point during a dive so whoopy-doo! I inflate (usually) before jumping, and immediately deflate all of it when everybody is ready to descend. If I did for some really weird reason have air in the bladder, there's a dump on my right shoulder to use... Or if not, suppose there's the breech ascent position for butt dumping.... I just don't touch the inflate button again until I'm back topside after the dive. If you don't put air in your BC, it will not frack up your buoyancy as you go down and up and back down and back up again whenever you want to and will it to be so. If for some reason I needed to add a little air to the BC when ascending, the mouthpiece is already in my mouth - I just need to hold the deflate button down and blow. Or momentarily remove the mouthpiece and hit the inflate button and then return the mouthpiece and resume.
C. You eliminate a hose and another dangly down thing if you use an Air2. Cutting the drag from that probably adds 1, maybe 1.5 kts, to my submerged cruise speed..... OK, maybe not. But still one less thing to futz around with.
D. I have taken to clipping on another bottle right under my left arm anyway, should I need it, like if a monster barracuda swam up and bit my LP hose off my first stage way back where I couldn't get to it to sip air. But that bottle is exclusively for me, not somebody else.