First off, I have to admit, I never actually tried this Rifftrax. I also have to admit that I generally have little patience for old B&W movies. As a general rule, I'm more into the special effects extravaganza type movies.
However, 10 or so years ago, I decided to rent this movie, soley for the purpose of being able to say I'd seen it. I fully expected to consider it boring and massively overrated. Very much to my surprise, I was dead wrong. I loved it.
I certainly don't fault anyone for not thinking this movie is all it's cracked up to be. However, there is something special about this movie. Even if I can't quite put my finger on what exactly it is that makes it so great, the fact that I, of all people, who rarely "appreciates" classics and doesn't generally care much for romances, thought it was great, means something.
Incidentally, I had a similar experience with Citizen Kane. Though, while I thought that movie was certainly good, I liked Casablanca far more.