There is something wrong here. Not beating you up and but there is absolutely no way for you to charge a fully depleted 3.7V 650mAh LIon battery with a 500mA 4.2V charger in 20 minutes or 1/2 an hour. It is not physically possible. Either your battery isn't fully depleted, or your charger is turning green early. Since you are experiencing this on multiple batteries, I suspect your charger is giving a false charged reading turning green before it's fully charged.
Additionally, this information is in the product description which I pulled from the joyetech website at
www.joyetech.com/ProductsView.Asp?id=70 :
"The eGo can be charged via our SUPERFAST USB CHARGER (1.5-2 hours charging), or you can mount the superfast USB charger to the inluded wall adapter if you are not near a computer."
Don't you think that if they could get it to charge in less than an hour they'd want to advertise it?
You did put together a descent, concise and otherwise accurate but limited review of the product - but claiming a full charge time of 10-20 minutes- has us all scratching our heads. The rest of us are getting more like 2+ hrs and to be quite honest, 2 hrs is a great time as is- its much faster than most other batterys- the important thing is that unlike the standard 510 battery you can fully charge a second eGo battery before running out of power on the one you just took out of the charger.
Other things you might want to consider in your review include comparing it to a standard 510, comparing it to other 3.7V devices, and comparing the standard atty to the LR atty on this device. Others have already done that and have gone to great length tearing apart eGo's, but you have to hunt them down because the forum search engine won't search on "eGo" in the text - it's too short ...
you may do an advanced search and put "eGo" or "joyego" in the tag as opposed to the text search and you'll get a start.....