If you find yourself dropping your ecig on hard surfaces (EVER), then watch out for the Joyetech eGo-C Twist. I've already had to repair mine twice. It's a genuine Joyetech too, not a knockoff brand. I actually find that the knockoff wins in the durability department. In fact I wrote a guide on how to repair eGo
batteries my blog (link is in my signature) after having broken 4 of them.
The problem is, the button has a flat side which fits against the frame of the battery connection on the inside. When you drop them, the button tends to move out of position (for whatever reason) and sink into the frame at an angle (which makes it non-functional). The only way to put it back into position is to pull apart your battery (I prefer to use an old Stardust Clearomizer for improved grip). The solder connection is so unbelievably weak that, if you accidentally pull to hard (especially when it's the first time it's been apart), SNAP! Now you have to solder the wires back together (sometimes requires stripping the tiny, practically hair thickness wires, definitely requires a steady hand, solder, soldering iron, and knowledge of how to get the job done/where the wires go). Good luck with that. After an hour or two of fixing a 5 second problem, you get the perfect storm and get it back together and it works again, until the next time you drop it. Then you gotta do it all over again. My recommendation is, if this ever happens to you, hot glue your connections to keep them connected.
My recommendation as to which device to get is actually not an eGo-type battery at all, if you find yourself dropping your ecig all the time. It's the VV Gripper, which is nigh invincible. I concur with the previous assessment that the more complicated the device is, the easier it is to break, and the harder it is the fix. I'm not saying don't get an eGo-C Twist, but I am saying maybe go for something stronger. Twists are sort of supposed to be for beginners, but they end up being more for advanced vapers who understand how to repair electronics.
P.S. Steeljan has a good youtube video on the subject as well, but she actually replaces the whole circuitboard inside the eGo battery, whereas only the most extreme cases require that procedure (and, of course, you'd have to have a spare dead battery on hand to cannibalize the circuitboard from in order to do the job).