Don't forget that another acceptable method of air sharing is to keep the octo bungeed around your neck (so you can find it very
quickly and hand your primary to your buddy. This also works if your buddy panics and forget's to tell you he's in trouble and rips your primary right out of your mouth.
And if they try to grab my bungeed octo, I'll nip at their fingers
Nightshade/TA: I actually use the same regulator I have as a primary as my octo. This is a Zeagle 50D. And to help with freeflow, there is a "switch" I can throw that keeps the venturi from "cracking" while in a current. The reason for this choice was that I desired to have the same performance in my secondary air source as I do with my primary. And my aim is to work toward dives that are a bit deeper and a bit more technical than your average recreational dive.
Another way of saying it is that I didn't skimp on my selection of a secondary air source. And I highly advise others not to skimp as well.
BTW, for an octo choice, investigate the Apeks TX40. These regs are getting very good "street" reviews and function well both as primary and secondary second stages.