in one word, straightforward.
I used this NPM package: OGhost
The only hiccup, was the out of memory error that caused the import to fail.
My Node.Js app was running with 128MB. I've increase memory allocation on Bluemix to 256MB, and the import went smoothly.
I found it funny to have migrated my 10 years old blog from Octopress on Heroku (set up that way 3 years ago when I was still a ruby developer) to Ghost on Bluemix, as I'm getting more and more into Node.js.
And this blog started as a Wordpress blog (but I still do a lot of Wordpress/PHP development).