I bought an oversized battery from ebay with a new battery cover for my N95 8GB and have been using it for about a year or so. The one problem i can say about it is that the connector terminals on the battery start to spread apart after a period of time so that they no longer hug tightly against the power terminals on the N95 so if i give the phone a sudden jolt or tap i get a reboot because battery gets disconnected in one of those three slots.

The temp fix is to take the battery and use a little pin to bend the two gold contacts in each slot closer together once every 2 or 3 months. But this is only a temp fix as eventually after a year those things suffer metal fatigue and the temp solution doesn't work and after about 4 days or so of bending those contacts back in you'll get the same issue again.
This is a quality control issue that needs to be brought up by the big corporations to the Chinese mass manufacturing plants in China that make these lithium batteries.
The metal on those contacts needs to be strengthened somehow or reinforced.so that 500 insertions and removals don't cause metal fatigue on the contacts resulting in loose power connection causing phone to become sensitive to sudden jars and taps.
After all, we live in the time of accelerometers, mobile tech should be expected to get jarred, tapped and have sudden jerks in motion.

I'm guessing Mugen overcharges for the same China mass manufactured batteries you can get off of ebay.
Anyone want to take a look at their Mugen battery and see where it is made?