I started out using the Joy 510 but those things only last a few months at most. Got tired of replacing them. I use volcano now. Much better quality, and the cartomizers last much longer than the 510. I order like 10 of the volcano cartomizers and it seems to last me about 3 months or more. Probably longer.
I've been using volcano for almost two year and I've been so happy with it that I haven't looked into other options. Surely, there have been more advanced models released over that time span.
Oh, and it you aren't making your own juice, that's something to definitely start. Huge money saver right there (like night and day) and very easy. If you want to enjoy all the favors and variety, knock yourself out, but you'll eventually get tired of spending the money and will stop caring about the flavor.
My recipe is roughly 45% Glycerin, 45% Propolyn Glycol, <5% Flavoring, <5% nicotine juice. And a touch of water.