As you have probably figured out, AIR 2's get flamed pretty hard - mostly by divers who have never actually used one or practrised with one - it;s just popular not to like them after hearing a story from your sister' friend's, cousin who knew a diver who had one once.
Ascending with one while breathing and dumping is, contrary to popular opinion not hard. The hose lenght needs to be long enough to let it sit comfortably in your mouth and it works well with either a separate right shoulder dump (as on most scubapro BC's) or with a dump integrated into the base of the coirrogated hose on the left shoulder (like most other recreational BC's). With the former, dumping has no connection with the AIR 2 and with the latter, lightly pulling or even pushing out on the hose between midway between the AIR 2 and the dump valve on the shoulder will tighten the cable inside the corrogated hose and dump air.
The only situation where you have to manipulate the Air 2 itself while dumping is on a wing set up for technical diving where the corrogated hose fitting is a solid elbow rather than a fitting incorporating a dump valve - and an AIR 2 is not really consistent with technical diving, so it is a non issue.
The Air 2 accomplishes the same thing as a bungeed octo in that it is very easy to find when you need it as you are constantly using it during the dive. They use the same basic poppet design as a R395 and R190 second stage, it's just powered by a slightly smaller diaphragm. Performanace however is comparable to the average octo.
In my experience the Air 2 is reliable in service if properly tuned and set up and will go several years without service (although annual service is a good idea as with any octo. The dual use as an inflator also ensures it stays flushed out and ready for use. People with maintenence or reliability concerns are in most cases victims of the shop not tuning the air 2 to their regulator and/or not setting it up properly with regard to lever play. If set up right, they are virtually bullet proof as long as you don't ignore them and rinse them after the dive like any other second stage.
A conventional octo, due to it's location and attachment can be harder to locate when your buddy mugs you for your primary (2/3rds of the time that is exactly what will happpen in a recreational OOA situation - all the pretty signal exchanging never happens as an OOA diver usually needs air right now as they usually exhaled just before discovering the reg is not delivering air and swam 20 feet or so to catch up with you to share air. They see your buybbles and want to make their own bubbles RIGHT NOW. It gets more interesting for you if, when you locate your octo, you discover it is full of silt, sand or sea weed. And Air 2 avoids those issues.
A bungeed octo also avoids those issues. However as much as I like using one, there are some problems for recreational diving. Most programs still teach using a short hose primary and donating the octo to an OOA diver despite the realities of recreational gas sharing. In that environment, a bungeed octo breeds some confusion. Also recreational divers are still taught to use and carry a snorkel. The combination of long hose primary, bungeed octo and snorkel all running around the neck and chin area can create an impressive tangle of equipment and the snorkel can interfere with proper deployment of both the bungeeed octo and long hose primary.
Personally, I think the long hose primary is the way to go. Once you share air that way, you will never wqnt to go back as it allows both you and your buddy to swim and ascend normally. It also tends to further reduce panic as the OOA diver is not having to deal with a short 22" to 40" hose with a second stage that is essentially "backwards" with the hose making a sharp bend constantly trying to tug the reg out of his or her mouth.
If I were king of PADI, they'd be teaching the use of a long hose primary and training divers to donate the primary tomorrow. It works infinitely better than the current standard. I'd also pretty much eliminate the mostly useless snorkel or at least relagate it to a pocket until it is actually needed.
At that point a bungeed octo around the neck would make perfect sense for a recreational diver. Until then, a long hose primary and an Air 2 is a pretty good way to go.
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Regulator wise, if you plan to dive a lot upgrading to a Mk 11 S555 or Mk 17 S555 makes sense as the inhalation performance of the current S and G series second stages are much better than the unbalanced R series second stages.
If penny pinching really counts, you can spoecial order the Mk 2 Plus with any second stage and yuo will get 90% of the performance of the MK 11 or Mk 17 S555 with the less expensive Mk 2 first stage. It is not balanced so the intermediate pressure varies about 20 psi during the dive, but the balanced second stage is not noticeably affected by this as is the case with the unbalanced R series second stages. Scubapro dealers usually will not tell you that as they make a lot more money with the standard packages as they are designed so that upgrading to a better second stage requires to also upgrade to a better (and much more expensive) first stage.
The Mk 16 first stage has been discontinued and so has the X650 second stage. When SP discontinunes a product, dealers are no longer restricted in the amount of discount they can offer - and this applies to the first and second stage combination so you potentially can get a great discount on whatever first or second stage is attached to the discontinued stage.
Both the Mk 16 and X650 are balanced and are excellent performers.
