I'd say Maemo 5 / the N900 is good for every "conscious" internet user. E.g. those who would install an internet browser other than internet explorer because they care to some extend. Previous versions of Maemo just weren't there yet, but Maemo 5 is.

Maemo 6 will be for everybody. At least that's the plan.

I wouldn't make this about open source or closed source, there is nothing that can be done with closed source software that Nokia can't do with Maemo eventually.