Another con: If you have a case for your N900, it won't fit if you have a Mugen big battery. I have a Mugen big battery and don't use it, primarily for that reason. Plus, con 2: It looks funny.

I use the spare battery strategy. And another poster mentioned the charge of a separate battery charger as if that was significant. It is if $10 or so breaks the bank for you, but then how did you buy an N900 to begin with? The advantage of a spare battery charger is that you can charge batteries while you are using the N900 and not be tied down to wherever you are plugged in; then it takes way less than 10 seconds to swap batteries once you get used to doing so.