After watching Seasons one through three of this show, I decided I'd hit a good stopping point in the show: the absurdity and character distortions had started to increase and the humour had started to decrease. But I found Seasons four through six on DVD at a garage sale for $1 each. It seemed like I had nothing to lose (I may have been wrong about that).
This season includes Raj and Howard accidentally kissing (episode 9?), a level of absurdity that wasn't remotely justified by the tepid laugh it inspired. Although as I'm now watching the fifth season, I realize absurd circumstances aren't quite so horrible as the character distortions (ie. our favourite geeks acting out of character) that became a standard in later seasons.
The list of (geeky) cameos is impressive (and entertaining): Neil deGrasse Tyson, Will Wheaton, Brent Spiner, Steve Wozniak, George Takei, and Katee Sackhoff.