I decide to buy one, came in today. Seems solid, the extended cover fits surprisingly well, and looks good too. Will report back with battery life after a few cycles.
Bought this one: http://www.ebay.com/itm/251124824283 the brand is named "Kevis".
The battery worked 20 hours of heavy usage (3 days) with factory charge.
Visual quality is better than Mugen's (I have Mugen 4200mah for g1).