I got an order of 5 extra capacity carts this week from AquaVaporCigs.com and I am very satisfied. I find I can vape pretty much all night in "almost pacifier" mode and MAYBE I'll have to refill once. The regular carts required 2-3 time more refilling. And since I don't vape much during the workday, a morning fill lasts until I get home at 6:00.
Overall, I am more satisfied with my RN4081 than my first e-cig (a Blu knock-off L88b). That one is good for vapor, but the battery dies in under an hour, vs. around 2-3 hours for my RN4081.
Both the RN4081 and the L88b are inconsistent vapor producers, though. Some days I can generate clouds of the stuff, other days I feel like I am sucking wet cement and all I get are a few wisps. I find I need to give one brand a break when that happens, and when I switch back, vapor is good again.
My only complaint with the RN4081 is that I have to really drag on the thing for a while to get a decent throat hit. The L88b has a more consistent throat hit, IMHO.
I thought about picking up a 510, just for the heck of it. But I dunno. I am happy with what I have, and part of my switch to e-cigs was to save some cash, so ...