I also agree that you're going to be very happy with the 510.
A couple things I've learned after making my way from a Smoking Everywhere Gold Edition 3.0 nightmare to an M401 experiment, to a DSE901 flood-fest, to the lovely Joye 510:
Forget cartridges. You are going to need e-liquid
Forget that junk about one cart = a pack of cigarettes. It's just not true.
Pick a flavor and a strength and order up. The 510 blows through 30ml for me in about two weeks tops (and that's being careful.)
A word of caution about going all out:
You might want to sample a few different maker's wares with smaller bottles before buying a big tub of something. I did not do that and have a couple big bottles of juice I really never want to vape again sitting here on my desk.
Batteries
It's already been said in this thread, but I just can't stress it enough. The 510 is a battery hog. With two batteries and one charger I've found myself in the last week in a pickle -- busting out my 901 to fill the gap while I let my two batteries charge up.
More than one charging option, and 4 batteries is a safe bet. I ordered two of the long 510 batteries last week when I realized that an hour of battery life was not going to cut the mustard.
Why didn't the Smoke51 unit work for you?
For me, I have been unhappy with everything I've tried apart from the 510 because of vapor production. Yes, the 510 produces a lot of vapor, but it's not smoke, and it's not like a cigarette.
I cut my e-liquid with vegetable glycerine (called VG around here) in a 10 drops of vg to 20 drops of e-liquid ratio and was so completely amazed with the results. VG creates beautiful, very smoke-like clouds of vapor.
Since I started doing this -- and I did it originally to reduce the amount of nicotine I was getting -- I've discovered that the vapor is a huge part of why I smoked. It's also a huge part of why I have been previously unhappy with my electronic cigarettes.
Adding VG to my liquid, I noticed that the 901 and the 401 work great. They are wonderful units as well.
I realize that my experience is my own, and I'm not telling you what to do. Hopefully you can glean something useful from my experience.
Good luck!