FWIW at least some of the knock off parts aren't terrible quality. One of my N900s I got from eBay turned out to be one of those Chinese refurbs. Screen, digitizer, battery, keyboard, etc all work fine.

I got a spare standalone digitizer, but the quality of that was terrible and requires much more force than an original.

The PolarCell battery for the N900 seems be as mostly good as the original battery, but the plastic on one end was brittle.