To me it seems like it really just depends on who you are, where you're coming form, and what your needs are.

If you want the latest greatest coolest gadget, then yes, you could easily end up waiting forever and/or be disappointed by how quickly any device becomes obsolete. If the N900 serves your purposes best of the devices currently available and you need something to carry out functions you can't currently do on whatever device you have, then it's probably a good choice; that is, you shouldn't get the N900 to fill future needs, you should get it because it fills current ones. If you're a user of the N810 and other previous devices, as BatPenguin discusses, then maybe the N900 isn't enough of a leap forward to make it worthwhile.

Once thing I wonder in addtion is who's to say that Maemo 6 will really be the end point? Nokia isn't going to just stop developing. Might not there be a Maemo 7? 8? 9?

All that said, I think it is legitimate both to get the N900 now for what it is and to be concerned about whether a platform is really mature yet. From what I've read, Maemo 5 sounds pretty stable and not especially buggy. But still, the difference between versions isn't always just about the newest features, it can also be about reaching a level of maturity and usability. Hello Windows 2000/XP vs. everything that came before it.