My first eGo device was from Volcano; their Inferno starter kit. It came with a 1000mAH battery and 650mAH passthrough battery. I like those guys because their service is pretty solid, and the warranty always comes through in the clutch (sadly, my 1000mAH battery has broken twice, but has been replaced both times).
The passthrough is awesome. I keep USB cables handy for it... 1 at my Macbook, 1 in the car w/ a USB adapter in the cigarette lighter port (because it's useless for cigarettes nowadays!), and 1 in my backpack. If you're stuck with a dead battery you can just plug it in... sometimes it won't work right away if the battery is totally dead, but after about 1-2 minutes it is functioning as a proper passthrough and is fully charged in about 90 minutes. Then you're good to go for at LEAST 6 hours.
The cap just screws on. The only complaint about the cap is that the paint on mine has chipped quite readily, so it looks like crap now. It's only use is to protect the USB port.
Get one.