The eGo = Riva seems like the best all-around off-the-shelf unit. Unless you were a half pack a day type smoker, like my wife, (for whom a 510 with 2 manual batteries is cigarette-like, small and practical), the cigar-like eGo/Riva/Tornado is the ticket.
There are more high-tech devices and "mods", and low resistance atomizers or high voltage batteries, and so on, for the hard core tweakers, but if you just want to chain-vape, the eGo/Riva/Tornado type device does the job. The 650 mAh batteries last about 6 hours of chain-vaping, and they recharge in about 2-3 worst case, so you keep cycling.
Oh, yeah, the 510 atomizers that are used in the Riva/eGo as well as the 510, work fine for everything with the standard 3.7 volt batteries.