I have been using one for the past 20 years and love mine. I've never had a problem with it and I do like the fact that I have one less hose to deal with. I will agree that it's not as comfortable using it vs my normal regulator but in 20 years of diving, I've never had to use it so it's not a big deal at all. Even if I did have to use it, most likely it would only be for a few minutes. So the comfort issuse to me is a non-issue.
One advantage I see is because it's the same as your inflator, if you did need to find it quickly, to me it would be instinctive to grab it without thinking because you are already use to reaching there for you inflator. So in an air share, you just grab it to use and donate your primary.
The bottom line is, some people like them and some don't. But for your typical recreational type of diving, to tell you the truth, I've yet to see a good reason why it would be a disadvantage to have it. Either way you go will be fine.