I've purchased several Mugen extended batteries for previous devices (N95 and N810) and had no problems with the batteries themselves, but the quality of the covers in both cases left much to be desired. In both instances, I had problems with the plastic latch hooks coming off or failing to adequately secure the cover to the device.

There is great potential for an extended capacity battery and a cover that fixes some of they issues with the N900's poorly engineered official cover. Namely, including a full device width kickstand with multiple lockable angles, a camera cover that doesn't actively interfere with taking a proper photo (haze effect) and make sure the cover lets the n900 lay perfectly flat and level on a desk. It would be nice if it was somewhat rubberized so that it stays put, too.

Unfortunately, even the revised version 2 cover that Mugen started shipping leaves much to be desired. Given my experience, I really don't trust them to engineer a cover I would want to buy.