Yes, I am happy, if not very happy with the Mugen's bigger battery.
The battery really is 2400 mAh, as advertised, no scam.
I've had the battery since February.

At first, they had a design flaw, that microSD-card and camera was not working normally because there was a small magnet and a white patch missing from the back cover. I fixed those myself, but later also got the new working back cover from Mugen.
I still use the old cover though, because it has extra space underneath where I have BT-dongle, USB-microSD-adapter and 2nd microSD card. The new cover has this space filled and "a hack" is not possible.

The battery gives just enough extra power comparing to the old battery, that I can really enjoy GPS-tracking, web-surfing through WLAN and other stuffs. It gives 180% time comparing to the factory battery 100%.

I don't think N900 with the bigger battery and Mugen back cover is too thick or big, when I remember what benefits I get with it. We just cannot go around physics. Li-Ion batteries won't get smaller per mAh.

The only problem is that N900's closed source BME is miscalculating the bigger battery capacity. It is either a bug in BME-system, or in Mugen battery's resistor.
See: https://bugs.maemo.org/show_bug.cgi?id=9789
In practise, this affects how you see your remaining charge in programs like BatteryGraph and BatteryEye. But still the phone will work and shouts for recharging when the voltage goes below 3.6V. The voltage measurements are correct fortunately in BME.

The back cover does not affect camera at all.
On the contrary, it is better, because it does not have the shiny silver frame around the lens which sometimes causes troubles with the standard back cover.

I do not have photos of the N900 with the bigger battery and back cover, but the Mugen's page has one photo. And then this is true on the page: "This battery adds 7mm thickness to N900. It will be 25mm."