I have mixed feelings about the movie. On the one hand, it was great seeing the guys up on the big screen. But...
First of all, the movie choice. "This Island Earth" is regarded as a classic of 1950s sci-fi. The movie was chosen because Gramercy Pictures, the studio behind the film, was afraid the public wouldn't go to see an intentionally bad movie like "The Incredibly Strange Creatures Who Stopped Living and Became Mixed-Up Zombies," or whatever it was called. Yes, there were some funny riffs, but overall, IMHO, TIE is a pretty good movie.
Secondly, the studio demanded that some of the more obscure references be omitted from the film. So a lot of the quips were dubbed over with more obvious ones.
Next, the studio wanted a "plot" to the film for some reason. As if three guys riffing on a movie wasn't enough. The plot footage was shot, but ultimately, much of it was scrapped, which is why in the finished film there are many holes in between the theatre segments.
I know this movie has cult status and quite a following, but it's just never worked for me. Too much tampering with the finished product...
And yes, Count, whenever Tom lost his head was funny. Sometimes it was unintentional, and Joel or Mike would put it back on. It seems that most of the outtakes from the show dealt with Tom's head always falling off!