Yes, you are completely mistaken, in my brief experience.

1. The battery seems to last forever.

2. I use an external battery charger, so I don't know whether the N900 detects the design properly or not or whether that affects the charging. The external battery charger cost me $10 a few months ago.

3. The battery and the cover seem well made. There is no cover for the camera and the external SD card doesn't work, but Mugen may well provide a fix.

I added now later, that although the battery charge level bars dropped from 8 to 1, the battery had been in use 25 hours after the last full charge. I was wondering in the morning already why the charge bars still show 8 although it had been so long after last charging.

So either Mugen has wrong logic in the battery, and it doesn't report the charge level (mAh) correctly, or then there is a hw/software bug in N900.

Anyway, ...well I still wait for a fix for the microSD-card, but battery has improved usability alot because if I have charged battery in the night, I can do about anything (except maybe not play Quake Arena 10 hours) during the day with N900 and the battery will have juice still.
I think Nokia should themselves provide 2400 mAh battery. This seems like a right size for the heavy user.

I won't recommend this 2400 mAh battery yet though, before they prove they have a back cover which makes both microSD-card and camera work OK.
I find it little odd, they do not tell about the microSD-card problem on the web-page.

yes, I thought it was kind of mind-blowing that I didn't at least get some instructions mentioning that the camera could use some protection. I suspect that Mugen didn't try it with a microsd card, which would explain why they didn't mention it!

In the last analysis, they probably made a mistake even offering this battery. But I sorta like mine, despite the defects.

There's no logic in the battery. There's just one resistor or NTC.
I was pretty sure that was an NTC to monitor the battery's temperature and prevent charging if it gets hot. But I could be wrong -- looking around a bit turns up references to both roles, and if it actually is a resistor to specify size, it would explain the wierdness.

I thought Mugen is powering HONDA Vtech Engine

Well, it's the next day and I'm still waiting for my Mugen to lose enough power for me to recharge it! Maybe it really does last forever! It certainly is nice to be fretting that the battery lasts too long for a change. This aspect of the new battery I like very much.

I just got a message from Mugen saying they are working on a solution to the Microsd problem and will send a replacement cover as soon as it is ready.

I measured the Battery Size Indicator pin with a dmm, and comparing it with BL-5C, BP-4L and BL-5J does make it look like it's properly reporting the bigger capacity.

Maybe someone with one of these batteries should file a bug in https://bugs.maemo.org/. Probably too late for PR1.2 though.

I found out that after taking a Mugen battery off and then inserting it and booting seems to fix the battery level indicator. But then rebooting again without removing the battery, makes the level indicators "broken" again.