They are popular entry-level collector bikes. Schwinn built bicycles with heavy 1950's manufacturing methods during a time when lightweight and performance was key. The Schwinn's could take more abuse than other bikes but the frame and fork alone weighed more than any other complete BMX bike, except for a Roadmaster. This manufacturing technique, combined with the fact that Schwinn grew fat from the success for the Stingray and Krate nearly two decades earlier, spelled the company's demise. The Predator (and upscale Freeform EX) and the Scrambler were Schwinn's only "true" BMX bikes. They used the old Stingray cantilever frames as long as they could.
A mint Freeform like this runs around $150, at least the last time I checked.