i purchased this last week while i was in hk. they delivered in a couple of days. build quality and fit is excellent. you can leave it on the phone and charge the battery, the phone will charge first, then the battery.
today i did a quick test - overnight i let my battery run down to about 15%. i then used the mugen to charge it while i was out and about. after 6 hours or so of reasonably constant use - music, email, whatsapp, internet, twitter, a couple of calls, the battery was back up to 55% or so before the mugen ran out of juice.