I have tried two old chargers with the adapter. The AC 2X, with the old style round plug, made the adapter very warm and although the N900 showed it was charging it took forever and I have my doubts that it was charging at all.

I am now using an AC 5X, with the newer small pin charger from the N95, and the adapter does not get so warm and it charges a lot quicker. Although the dedicated microUSB charger is still the quickest by some way.

Charging from my laptop with the MicroUSB takes about the same time as the AC 5X.