The father of my buddies dives with an inflator/octo device and seems to love it. I, on the other hand, decided it wasn't for me.
Anyway, earlier this year, I was at some springs with one of his daughters (a decently well-trained diver with varied experience who was my buddy in our Rescue course last year). On the last dive, we decided to play around by practicing as many skills, routine and obscure, as we could come up with. One of them in particular proved quite enlightening.
Each of us, in turn, went "OOA" and accepted the other's alternate. Then, while sharing air, we made a full circuit of the spring basin to simulate the real-life situation of having to go back to an anchor line after an incident down on a wreck. Once we arrived back at the beginning, we swapped out and repeated the exercise sharing the other's gas. As you can see, it was a rather trivial skill, but as I said, it was quite enlightening.
The first thing I realized was that given the standard octo hose length, in order to breathe from her alternate while swimming side by side, I had to flip it upside-down in my mouth (which wasn't very comfortable) or swim along directly above her (which would impede communication precisely at a time when it is most important). My alternate isn't the standard second-stage shape, so that wasn't a problem with mine (although, admittedly, it's no sweet breather).
What *really* surprised both of us, however, was how little length a standard octo hose provides. We successfully made the circuits, but it was neither simple nor very comfortable. The thought of trying the same thing with the common primary second-stage hose length was downright laughable. If one of us were hugging the other (or the other's cylinder, as the case may be), perhaps we *might* be able to make some sort of headway, but it would be immensely more difficult.
We discussed the skills after the dive, and the air-sharing got much discussion. We basically agreed that the standard-hose-and-inflator/octo concept is simply not acceptable. It'll certainly work for making a direct ascent to the surface, but trying to make stops while that close would be... interesting..., and it would not be practically possible to proceed any significant distance to the anchor line. (Even on a wreck at 80', if there's a ripping current, you can be *quite* far down-current by the time a safe ascent breaks the surface.)
Using an inflator/octo together with an octo-length primary second-stage hose *would* be
adequate, we decided, but it would require significant practice in order to be reasonably skilled enough for buoyancy control while breathing the inflator/octo in emergency air-sharing ascents. Practice would also be required in order to be able to properly handle manual inflation. (We've seen too many people have trouble with that skill and inflator/octos, even on the surface. They are simply more complicated to operate than plain inflators, as is readily obvious, so they require more practice.) Additionally, to breathe an inflator/octo, it must be in your mouth, but to exhaust gas from your BC, it must be above your BC. Swimming horizontally while controlling your buoyancy and breathing is therefore rather more complicated than it need be -- it's not an issue for a direct, vertical ascent, of course, if those are all you'll do or need to do.
We both decided to continue diving a "normal" inflator, primary, and octo configuration, although I always dive a pony (on anything over 19' and even on many things shallower), and the idea of a somewhat longer hose with which to share air is something we're certainly entertaining. Still, we could see how an inflator/octo-type configuration could be workable.
-- Personal commentary follows. Ignore if desired. --
My personal opinion, if I may mention it with all due respect, is that while there are certainly reasons a diver can choose to go with an inflator/octo, the whole "streamline yourself by eliminating a hose" approach is utter hogwash. Removing one narrow hose won't make a significant difference to your drag, and if anyone wants to rig up a tow-dummy to quantitatively show otherwise, I'll post a YouTube of myself eating this post (printed in non-toxic ink on organic paper, of course).
Decking students out in poorly configured gear with octo hoses connected such that they make giant flying loops off to the side is quite common from what I've seen, but all it takes is two minutes with a hex key and a crescent wrench to correct that lack of forethought and let the hoses lie where they ought. (The formerly-rental reg set a buddy of mine had was actually configured in the geometrically worst *possible* configuration of hoses in that order on that first stage. Simply moving two hoses changed it from a jaw-fatiguing primary and hula-hoop octo hose into a comfortable primary and a side-hugging octo hose.)
Some of the most skilled divers I've met have had long hoses, drysuits, can lights, or all other manner of unstreamlinity. I consider it an example of the worst in the "scuba industry" to tell people they can become better divers by trading out a hose and a wad of cash to "improve their streamlining". I don't mind people having inflator/octos. I not only *own* a pair of split fins, but I even understand which dives they're for and which dives are better served with my big lumps of black rubber. What gets my goat is simply when innocent divers are sold gear as snake oil for problems that don't exist. (The snake oil might be *excellent* for a nice stir fry, but it's not going to make your hair come back.)
Well, anyway.
-- End personal commentary. --