If you're happy with Maemo 5 functionality I think you should buy the N900. It's a nice phone. Don't count on Maemo 6 though. You will hear lots of people talk about stuff that's being worked on. It's great but whether or not the final product of these efforts will be usable by normal people rather than developers is the big question.

There will be a few non-supported releases for the N900, a hacker's smörgåsbord if you will, but if you are an end-user that wants Flash, OVI Maps, Skype and all the other nice things that come with a finished platform, don't expect anything other than stock Maemo 5. If you get your hopes up then you will likely be disappointed.