I've migrated from Octopress to Nikola.
Nothing personal. Octopress is fine software, but:
External packages, like themes, made my installation quickly impossible to understand and manage. What's the difference between
style? I don't know.
Performance. Nikola builds sites nearly instantly. Even with a moderate amount of pages, I found Octopress too slow. Maybe I just need a better laptop.
Python. While I'm sure the Ruby tools are of similar quality to Python's; I know Python's tools. Porting to Nikola was easier than learning about A-grade Ruby developer installations.
Anyway, I'm on Nikola now. Pretty happy with it. I used nikola-octopress-import. Did most of what I wanted; I contributed some minor PRs so that it would also handle extended date formats (with seconds and timezones) and legacy HTML posts.