I spend weekends alot in the summer in a place with 2g-only (not even 2.5g). We have wifi in the house, so when I am home everything works like a charm. When I go out and about, 2g is pretty slow for most stuff (like mapping, web browsing, etc.), but it is functional if needed. Text-based stuff like IM and background stuff like email works mostly without any noticeable difference on 2g. Even Facebrick runs fine on 2g, though the facebook site itself is pretty slow via web. And the phone automatically switches to WIFI at home or wherever else you set it up, so you may get good coverage most places anyway.

The other thing is: if there is no 3g where you live, any smartphone is going to be slow. So the N900 will be no worse than anyone else at 2g.