My routine has not changed in months, and I monitor my battery usage closely. I start the day (full overnight charge) reading mail, RSS feeds and BoardExpress, for 2hrs+. At 2hrs I always check the battery: HTC, about 55-60%, Mugen, about 45-50%. I tried to monitor all of my uptime while using the Mugen, and came in at a little under 4hrs before the low battery indicator kicked in. Previous tests with the HTC got me to around 4.5hrs. Not scientific, but I did expect something close to a 22% boost, and at very best got no more than I already have. My typical strategy is to buy a spare OEM battery when I get the phone and switch out every three months, so they both wear evenly.
And to answer your other question, I am no expert, but basically you can't put 10lbs of crap in a 5lb bag. You can get some battery test results at batteryboss.com and form your own opinion. I do believe that larger third party batteries (with extwnds battery doors) do hold more juice but these "same size, more juice claims" are no more than claims. But by all means, test for yourself.
Sent from my HTC Surround using Board Express