I've been ignoring Rifftrax for awhile now it seems. When I first found it, it seemed so awesome, yet after watching several of them it seems like the jokes all run together... it's hard for me to describe, but it also seems like the style of jokes is the same for all the movies I've seen. They're mostly just the standard "comment on something in the movie"... there doesn't seem to be any of the actor-resemblance jokes, bombast jokes, character impersonation jokes (though to be fair a lot of those went out the window in MST with Trace's departure), references to other works the actors have been in, etc.
It's hard for me to describe, but it just seems like a lot of what made MST fun is sorely lacking in most of the Rifftrax. There's only a few Rifftrax I would watch a second time