Rock N' Roller Coaster is, by far, my favorite ride in not only WDW but ever! I'm a huge Aerosmith fan (what can I say? My mom and dad raised me on classic rock. My next dream would be a roller coaster set to the Doors or Queen!) and I didn't even mind waiting in line because they play Aerosmith music the entire day. The music throughout the coaster track is really loud, but it's perfect. It's not at all scary, either.
I went during this past summer and the wait was about 60-80 minutes and I knew that I'd want to ride it again so I got a FP, waited in line, then went right on it again. FP is probably your best bet if you're going in summer.
I'm not sure of the best times to ride it, I've never done EMH yet because I haven't been there since they started it! But on the next trip, this Saturday!!!, I'll be sure to check out the EMHs and stuff.