rpm has one of those fancy tank jobs, i've never tried one. looks pretty cool but if the size is near that of a cigar, that would put me off. the tank models do look promising though.
all of the basic retail ecigs recommend the battery be replaced every 6 months or so (depending on use), i've gotten 8 months out of an efox before it quit holding a charge. generally speaking cartomizers (atomizer attached to the cartridge) are more expensive than standard cartridges (atomizer attaches to the battery). with the cartridge models, the cartridge can be refilled fairly easily and there are a lot of videos on how to do it. if you're going to refill rather than buy pre-filled, out of all the smoke juices out there, the johnson creek smoke juice is the best, u.s. made, not made in some southeast asian garage. there are a few other smoke juice companies that sell u.s. made stuff but i've never tried them and i don't know anyone who has.
i've tried 3 retail brands with either cartomizer or basic cartridge, efoxecig, blu, and square 82...of the three, the square will hit your throat like taking too big of a drag off a filterless cigarette. i have a coworker that is using the njoy and he loves it.