In one of his stand up routines, Chris Rock talks about shopping malls: there's the mall white people go to, and the mall white people used to go to. Japan's amusement parks seem to follow a similar theme. There are the amusement parks people go to (Disneyland, Universal Studios, Fuji-Q) and the parks people used to go to (almost every other one). Japan saw a great period of growth in the bubble economy. New parks sprang up all over the place, and older ones expanded quickly. Unfortunately, many of those parks have since failed, and those that survive sometimes seem to be barely clinging on.
Many parks are doing okay, of course. During weekends and vacation times, the crowds pack in, and without any new development the loans get paid. But unfortunately, most of them haven't really added anything in years.
Access is through Iwamizawa; a discount ticket is available that includes round trip transit (Sapporo -> Iwamizawa -> park), admission and a free pass. Alternatively, you can take a train to Iwamizawa and the bus to the park. It takes about 60 to 90 minutes to get to the park from Sapporo.
Be sure to check the bus timetable before your visit, because they don't run very frequently; also note that if you want the discount combo ticket and you want to leave early in the morning, you should buy the ticket the day before from the Chuo Bus office at Sapporo Station.