They're not bad, just weird and if you're in a good mood you will enjoy them. In fact, they're better than some (Hollywood) movies that are supposed to be serious.
Generally terrible is not a very strong argument, so as I said, if you want to enjoy them you can. I don't know what Grindhouse is.
I would only see one with a recommendation from someone I trust. I prefer to watch films that are more than enjoyable; films that have the potential to make it into my top one hundred. They are the team who made the Grindhouse double feature. They have created such masterpieces as Machete and Hobo with a Shotgun.
Planet Terror. It was in the original double feature. I still haven't watched even one of them fully. I need to get a hold of them soon.
Deathproof is terrific, such a build of suspense and tension. Planet terror's more in it for the gore factor, still fun though.