Yes, I have to say the same.... sometimes it shows 20-30% left but then in the next minute when I don't look.... dead phone!
HOWEVER, per yesterday I have to make another statement: I am not selling the mugen 1800 just yet!
The day before I was outside (29°C) and had my phone in the shadow(!) with the music/mediaplayer running via bluetooth. After about 3 hours the battery (from full!) was down to 50% and the temperature was reaching as high as 53°C(!!!!!) of the battery/phone (per batti 2.4)! I was using the stock 1500 orig. SE battery!
I call that fracking hot!
Now, today, I did the same with the Mugen 1800 and the conditions were all identical!
But the battery was never going over 33°C (!!!) and the power was only down to 60%!
Now, since that mugen can die with 20-30% showing as left, does not mean a lot. However, the temperature is for me at least, worth it ! Especially since I am convinced that this heat issue is at least one cause of those cracks, particularily under the volume button where is gets the warmest/hottest!
So for now, I will use the Mugen after all. It is definately not as I expected it to be but considering the lower temp, make it worth for me!