Today I found myself stuck with a server having the FacetedSearch v 3.0 installed
on it, along with the v 2.5, and as expected it has some conflicts, and after an hour
trying to fix it, I decided to reinstall everything.
A few minutes later I had a clean install of v 2.5, and it seems to works fine, except
for the SearchFacets WebPart kept giving me an object reference not set to an instance
of an object. After beating myself for another 30 minutes I realized that the WebPart
properties where all blank. So after changing them (important one is the one about
the scope, it has to be valid), it all worked fine.
So after some testing, it seems that whenever you have the .webpart files already
in your library they got replaced, but without any default settings in it. And that's
something that is nice to know, and can save you quite some time.