I loved MST3000 and miss it ! I bought 2 of the Rifftrax, Night of the Living Dead and Plan 9 from Outerspace. I have only watched the latter so far. It's pretty funny. I actualy prefer it without the comedy bits during intermissions.
It looks like most of the Rifftrax are well known or cult classics, not bizzarre B movies no one ever heard of like most of the MST stuff. Plan 9 is almost too easy to rip on, but I would guess that it's easier to sell a dvd of a single movie if it's a well known movie, where on the tv show it's ok to use an unknown movie since you aren't buying it