I'm happy with the results I've had at 12"x18" - it's just that they don't stand up very well to the natural human reaction to inspect them up close. There's no pixelization, they're just not as sharp as you might wish for.
One thing I have noticed is that the subject makes quite a difference to the overall perception of the print: a finely detailed landscape will look a bit 'soft', whereas a closeup of, say, some flowers will look much better.
I was very interested in BJ's comments about enlargements from film on this thread: https://www.nikonians.org/dcforum/DCForumID21/46.html#. I wouldn't expect the D1's output to look as good simply because the film can hold a lot more information, but on the other hand there'd be no film grain to worry about - the D1 is capable of producing very smooth images. Ultimately it's hard for me to say just how good these prints are since I've never tried to produce an equivalent print from a high quality scan from film.
If you'd like to have a go with a D1 NEF then let me know and I'll upload a suitable one for you to try.