I have had the mugen battery for 2 weeks now, so I thought I would give my thoughts.
I had the verizon extended battery from day 1, so I am comparing to that.
The battery is slightly bigger than the vzw extended. The vzw extended cover will attached over the mugen, but does not quite snap tight.
The mugen cover is a little bigger, but not much. I like the stand and have had no issues with it.
Battery life is interesting. The manual says for the first 4-5 charges charge format least 8 hours and then do not charge until the phone complains of low battery. Sounded a little strange for a lithium ion battery but ok.
1st day - full charge over night. phone done to 15% by dinner time. Grrr.
2Nd day - full charge over night. Phone done to15% about 9pm.
3rd day - full charge overnight. Phone at 50% by 9pm :)
Now I finished a normal workday at 83%. I also managed not to put it on a charger until 6pm the next day.
Cases: i had an thin case with a cutout back for the vzw extended. It fit over the mugen not quite snug. I set my phone in the sun for 10 minutes and the case snugged up. Fits great.
The two small holes, one on bottom and one top left side were slightly cover by the case. I enlarged the holes in the case slightly and good to go.
So far very happy. I will update as time goes on.