I too thought X-Men was good, not a classic, but a fine piece of entertainment - and I look forward to a solid riffing.
Personally, I hope Mike & Co. present a mix of films - from clunkers to classics. Just as on MST3K, the bad films should be riffed because they are cinemtic dregs and deserve the razzing. I think good or classic movies will make good riffing fun because it's a chance to have some fun with some famous characters and actors.
For example, I would love to hear riffs of
Citizen Kane
2001 (There is so little dialogue, it would be a riffer's dream)
Gone With the Wind
Taxi Driver (I'm still trying figure out why Travis Bickle took Cybill Shepherd' character to a porno film on their first date - I've read some theories on this but I don't buy 'em)
Apocalypse Now (Oh, the Brando and Dennis Hopper jokes!)
I could list many, many more.
So, I say keep the Roadhouses and Crossroadses coming, but throw in some quality films on occasion as well.