I purchased vB4 on the premise that the new search engine would work for big forums, after playing with it and inquiring with "the people" I have heard otherwise. Even the search on their own site with vB4 is slow and it is no where the size of my forum. If that is the case, I couldn't go back to vB search even if I wanted to ... my forum would come to a grinding halt.
So while I do have vB4 purchased... I'm currently in a holding pattern.