Several people have migrated from Wordpress into Nikola, and here are some of their descriptions of the process:
A slideshow by kopf (hey, that's his IRC nickname ;-): Migrating from Wordpress to a static-page blog
A tutorial in german by Niko Wenselowski (who has done a lot of work to make the Wordpress importer behave): Wordpress nach Nikola umziehen
Slightly older, this post by Kushal Das: Importing your Wordpress blog into Nikola
In general, it seems to be working, but there's some work still to be done. Wordpress supports many different plugins and extensions which react to markup in their pages, and supporting that's almost an infinite task. Currently Nikola's importer handles a few of the more common. But if you try to import your blog and get less than ideal results, please file a bug and I'll do my best to fix it.
Usually the fixes are rather simple, it's just that I have never seen that specific thing ;-)