This past weekend, I had a flood of over 100,000 of attempted comment spams. The bayesian filter stopped all but 5 from getting through. The main effect that I noticed was that the database queries to run these spam check was hitting the database hard.
Bad Behavior is a collection of php scripts that is intended to stop these http accesses before the queries reach the database. It analyzes the actual HTTP request and compares it to profiles from known spambots.