The only real con for us would be potential for neck injury. However, by the time I introduced it when he was about 14 months old, I had already spend a considerable amount of time actively training LLW, so he knew the drill.
He didn't love me putting it on him, but he's very hard physically, so most things like this don't really bother him. He's not a dog that needs slow introductions to things. After about a month of use, he's really excited when he sees me getting the head harness out.
I think when using it you have to be 100% focused on your dog, to ensure they don't lunge and injure themselves. I would never have him wear it while doing another activity because of the potential for inadvertent serious corrections.
I also never actively correct him with it. I just hold the leash still, bracing against my hip to ensure he doesn't get inadvertently corrected. But it offers me a way to very to very easily control a 90 lb, very strong dog.
It's the best tool I've ever tried (and I tried several no-pull harnesses, and a prong collar).