Sportscasters are relatively common for the most part. Lots of undesirable "common" cards of weird sports with non recognizable names or no people featured, just equipment or other non-human subjects. Being oversized compared to traditional card sets didn't help their popularity. Interesting set though if you want to learn about other sports. Obviously the more mainstream a sport, the more popular the cards would be (baseball, football, basketball, hockey and to some extent golf, boxing, soccer, etc) and any bigger name players generate more excitement. As yankeeswin mentions, the higher series cards issued in 1979 near the end of the run seem to be a lot tougher to find and much more expensive, including the Gretzky, Johnson and Bird "rookie" cards. Like anything, subscribers eventually lost interest and cancelled and those that hung in there until the end were rewarded later (if they held on to them).
I like them myself and probably have 400-500 or more. I bought a very large lot on ebay years back that came with dividers and a few booklets too and then I have pieced together many of the major sports cards as well and I even have a few unopened packages.