I visited the Rock and Roll Hall of Fame Museum on a trip to Cleveland this week, and I had a much better time than I expected. In particular, I learned a lot about early bands and singers I hadn't really heard much about before, such as Little Anthony and the Imperials. While there are certainly fair criticisms of the place with regard to who's been inducted and who hasn't -- influential acts like King Crimson, the Smiths, Sister Rosetta Tharpe, and Kraftwerk have thus far been left out, and in my opinion Edgard Varèse really needs a mention in "early influences" section of the museum -- it's a neat place to visit if you're any kind of pop music geek.
There is a whole lot to see in the museum; the bulk of the exhibits are on Level 0, a basement level that obviously isn't visible from outside. The upshot is that the museum is much larger than you think it's going to be as you drive past the pyramid-shaped building. They have special exhibits that rotate in and out; there was a whole room dedicated to John Mellencamp, which was fine, but given their recent deaths I would have preferred in-depth exhibits on Prince or David Bowie. The museum advertises a rap exhibit but I found it rather slight compared to other exhibits.
My recommendation, if you can only go for a single day and want to see as much as possible, is to go on a Wednesday. The museum is open from 10am to 9pm on Wednesdays and Saturdays and is open 10am to 5:30pm on other weekdays. So you get an extra 3.5 hours to wander around, and the advantage of going on a Wednesday is that the crowds will be much thinner, apart from the occasional rowdy school group that rushes through.
We arrived right at 10am and stayed until 9 and managed to see all the exhibits and watch all the movies they were showing in the theatres, but the special exhibits on the top two floors of the building were closed, and if those had been open we might not have been able to see everything. We did not listen to all the recordings; I think there are something over 48 hours of recordings available in the various exhibits.
The cafe is on Level 3 and like every other museum cafe I've ever encountered, the food is overpriced, but I was impressed by the quality. Your mileage could certainly vary there.
The museum recommends that you park in the garage for the Great Lakes Science Center, which is right next door to the Rock and Roll Hall of Fame Museum. Parking is about $10, and leaving your car there is convenient if you want to hit both museums in a single day. Be aware that the science center is not open late on Wednesdays, and so you will have to walk around the building to retrieve your car, which may be an issue if you find yourself dealing with sub-zero wind chill due to a gale off the freezing lake, as we did. Fortunately, the walk probably isn't quite long enough to result in a frostbitten face, but be prepared. The Rock and Roll Hall of Fame Museum does have a free coat check in the basement.