I'm using a polarcell (I think it's the same as the one in the link) and it is quite ok. I've bought it some time ago, like 4 years and it's still good.

I'm not a heavy user of the N900 in a daily basis (just a few short calls, some texting, some music, some browsing) and it lasts like 2-3 days, so for someone who uses it heavily it should last a full day.