I've been writing Java code for a long time and I never thought about making a class or a method final until recently. Actually the only place the final keyword was used was when declaring constants. I still don't think you will gain much on performance by using final but the biggest selling point is the correctness it brings to the code.
So far our development team has not looked at the correct usage of final keyword during peer reviews, but going forward it will be added to the peer review guideline document. Hope I'll see more finals in our code after that [if anyone follows the guideline that is :) ]
Anyway here is a link to a good explanation on final keyword: The Final Word on the final Keyword