Howdy Armyfrog;
I'll give you the scoop on the 3 piece. (Battery, Atomizer, & Cartridge.)
I mainly use the Model 510, which is very popular, has good flavor, and good throat hit. I started out with the unit that looks exactly like a ciggie. Soon the novelity wore off, and I went to Mods. Basically, a Mod is your normal atomizer, and cartridge screwed into a larger battery for more life. In addition, you may want a passthrough also. A Passthrough is a small unit without a battery, & has a 4 ft cord with a USB plug in which you plug into an adapter for 110v / 12v / or USB computer hub. Passthroughs are great when your not on the go, and you can save your batteries.
2 types of batteries. Auto, has a tiny vacuum switch wich activates when you draw on it. Manual, has a tiny button you press as needed. Personally, I think the Autos foul easily. Manuals are sealed, and the juice won't leak into the working parts.
Don't get taken in by people who try to sell you replacement cartridges. We purchase the juice, (E-juice / Smoke Juice / etc.) and refill the carts. Much cheaper. And after you get the hang of it, you'll be mixing your own like most of us here.
I'm over 2 years now, smoke free. Don't take my word on the 3 piece as "Carved in Stone" Check them out, but wait to hear from the guys who use the 2 piece Cartomizers, and the drippers before you make your choice.
In the meantime, you can check out these links;
Examples of some 510 Starter Kits;
510 Kits
The Mod that I use; (As mods go, it's very large, smaller ones are available.);
I have The Goldrush Mod;
Custom Mods
Examples of USB Passthroughs, and 110v adapter I use;
USB Passthroughs
A few places to get juice;
E-Liquid
Johnson Creek Original Smoke Juice - The Best E-Liquid in the World - Made Right Here in America
Smoke Juice
I'm not pushing any partucular site. These are only a few that I personally deal with. Other members will give you more sites to check out.