I don't have much experience with different models, but I would definitely recommend an M-40x model to anyone.
I started with a Green Smoke cartomizer unit before I found this forum, and while I liked the idea, that particular model didn't work well for me.
I do love my M4's. Unlike people who talk about attys being disposable, I have only had one atty go bad on me in over a year of part-time
vaping. I've bought extras, but mostly for keeping different flavors separate.
I really got excited when I got my M-402 PCC. It will charge a battery, hold 4 carts, and hold an assembled M-402 PV so all you have to do is open the case, grab the PV, and start puffing. Just like taking a cigarette out of a pack.
I can't speak to how my M4 compares to other models in vapor production, etc., (and actually I do want to try something new some day) but I know it works great for me, and I like having the compact cigarette-pack-sized box that contains everything I need for a day or two.