I have the Mugen Power 3600mAh battery (#HLI-N810XL.) From what I've been able to gather, Mugen seems to have a good reputation. The battery is a little bit thicker than 2 original BP-4L's stacked up together. It fits well into the battery socket space. The empty space created next to the battery (under the add-on cover) could be used to stash something except that the inside of the cover has some strengthening ribs running across it. Even so, I managed to wedge a couple of mini SD cards in between the ribs. This after-market cover is plastic, but it is shaped nicely, fits very well, and is painted the exact same gray metallic color as the permanent piece that runs right above it across the top of the Tablet. In place, the new cover is about the same thickness as the main keyboard section of the Tablet, maybe a hair thicker (I didn't break out the micrometer.) It does not get in the way or impede the operation of the kickstand at all. When holding the Tablet, I always open up the kickstand and slip one hand up between the back of the tablet and the kickstand. This is still possible with the new cover, although it does get a little tight around my knuckles. But it's still a very doable way of holding the tablet. Oh, and the Tablet still slides into the original pouch, although it's a tight fit.
OK, battery life. Actually, I've never put a clock on it. I use the Tablet sporadically and for a wide variety of uses.......some which are much more power intensive than others. But sure, I would guesstimate that the 3600mHa lasts at least twice as long as a new 1500mHa battery.