I have been to both the Punchline and Cobb's Comedy Club in SF, and have to say I have always had a better experience ant the Improv in downtown. The seating is better, location is better, and we've consistently been getting top tier acts month after month. If you've never been to a live comedy show before, this is the place to go. You'll probably be coming back for more.
Here is a list of some of the "special events" coming this summer (Improv's way of saying: "these our our big name acts"):
Jim Breuer: Jun 25-28
Josh Blue: Aug 14-16
Kevin Nealon: Aug 21-22
Eddie Griffin: Sep 11-12
Gabriel Iglesias: Sep 25-27
I've seen Jim Breuer, Josh Blue, and Gabriel Iglesias... all were amazing. By the end of Gabriel Iglesias' routine I was laughing so hard I was almost in tears. Gabriel also gets respect for brining in San Jose Lightrail and pedicabs into some of his jokes.