My last N9 was damaged (my mistake) and I went straight for another! I love it and I would use it till the end of times if that were possible. It's imho even more intuitive in some aspects than the upcoming sailfish OS (what I can tell so far....)

However, be aware that harmattan is dead in the eyes of nokia and only receives support through the community. That means no bug fixes for mail, browser,.... as these parts are closed source!

Also, new features will only come to the N9 via the community devs.

As for the performance, mine is fluent in everyday use and pretty reliable as long as you don't modify it beyond your understanding....