The e-Power is a step up from a kGo, mainly due to the replaceable batteries and switch. The BB is a big step up from the e-Power in terms of quality and durability and reliability, but it's 3.7V, so it won't be vaping any better than the battery you put into it, just like the e-Power. It's smaller than the e-Power, but the biggest 14500 battery you can get is a little smaller than the 14650 battery an e-Power takes. Actually the batteries are the same diameter, but the e-Power's battery is about 1/2 inch longer.
The other thing about the BB is the feel of it. The switch, the weight, the overall feel of holding it. It feels like a really high quality tool. It's not too fat or too thin. It's just big enough to be comfortable in your hand and small enough to completely conceal in a closed fist. The button is the perfect size, it's in the perfect spot and has a great feel to it as well. I bet I put a thousand cycles on my button just because I liked the way it felt when I pressed it. Weird, I know.