The keyboard is the deciding factor
I have the N97 mini, and basically it's as this review says - fundamentally great phone but using it can be a little awkward due to touch screen and OS. But I still like it for the simple reason that the keyboard makes typing a much more productive affair. Not only is it faster (because you make fewer mistakes) but you can you see the full screen while you type.
So that's the bottom line. Why would you want to buy one? Well, hi-res screen, voice navigation and hi-res camera are all nice to have, but not deal makers. But if you send a lot of emails, comment on blogs, or write on twitter whatever - anything involving text input - then the slider format of the N97 suddenly makes it a very attractive option.