I used an ebay chinese 3800mAh battery in my Spectrum from June 2012 until last week, when I first got it (the spec comes with a 2100 OEM) It extended my battery life by 18 hours (from 18 hours) of moderate usage (less phone calls more display on, switching between multiple apps, playing a game for 2+ hours non-stop, reading books, 4G always on). Last week I was still able to get over 24 hours of use from the phone, but with less overall activity (10-30 minutes of phone call, 30-60 minutes email reading, 20-40 minutes forum reading in tapatalk, maybe 20 text messages, with 4G and GPS on all the time except airplane mode while sleeping). If I used the phone even less the battery could go for 1d 22hrs and some change with 15% left after two nights of airplane mode. I stopped using it, because the back cover it came with has worn out and will no longer stay on the phone. Due to the size of the battery and it's weight, this means picking the phone up too fast would cause the battery and the back cover to stay behind...
total cost $9.99 (free shipping)
better than paying $45-$65 for an OEM or Mugen or MobileCityOnline.com, definitely.
did I maybe miss out on 2 or 3 hours of battery life, possibly, is that worth the extra cost, no.
do I wish I had the OEM back cover, definitely (came with a kick stand build it!)
don't get me wrong I intended on buying the Mugen from Verizon (3500mAh), as they were half price in store only, but when I went to the store to get one, they told me they never heard of it and when I showed them the item number they said it was in the system, but no stores within 100 miles had one, none of the warehouses in that same area had any either, sorry pal. I even tried when I went on vacation about 165 miles from home, and got the exact same story. I even called Verizon to ask them to ship me one at the sale price, they said no, in store only (inventory closeout, they would not be carrying it anymore). So I wanted the OEM and was forced to buy the knock off as I was not paying $50+ for a battery to put on a phone I got for $20. (If I had waited one more day I could have gotten the phone from amazon for $0.01)