Scosche goBat II, Tecknet is a third of the price
Just back from Glasto, used the TeckNet Dual-Port iEP380 5000mAh
http://www.tecknetonline.co.uk/products/TeckNet-Dual%252dPort-iEP380-5000mAh-Universal-USB-Battery-Pack-For-iPad%2C-iPhone%2C-iPod%2C-Nokia%2C-HTC-and-Others.html
This kept my Orange San Francisco topped up for 5 days.
At £21 and very similar design, I'd say it was a better choice