Need: I found that even with a perfectly working PV, there's still the urge to smoke, no matter what nicotine level you take in. I'm almost certain that all of the additives in (most) commercial cigarettes are put there for the sake of keeping you addicted. I've found that having the PV helps with the NEED to smoke, as well as easing side-effects of nicotine withdraws.
That being said i haven't had a cigarette in about 6-7 weeks.
I have some flavorless 36 from Totally Wicked, and some from Vapor Nine (i think, its unlabeled) and can say that its a nice change from all of the flavored stuff. It may sound weird, but its a very "clean" vape. You can definitely tell there's nicotine in there, from throat hit, and the "peppery" feeling/taste of the nicotine, but sometimes i get headaches from some of the flavors out there.
As for the carts, although the M series has pretty much the smallest cart size of all the models, it also has the amazing ability to "pull" most of the liquid from the cart. In the 901, when i pull the cart off the atty, I find that there's still a good amount of liquid left in the cart. When i do the same with the M, most of the filler is practically dry. In essence, while it may not provide as much vapor or throat hit as some other units, the M is VERY liquid efficient, as well as reliable, and IMO pretty much the sexiest PV (outside of some very nice looking mods) .