I have been to both Great Americas in the last year:
My opinion - Six Flags is MUCH MUCH better.
My reason - Six Flags really surprised me. The park was beautiful, well themed, and very impressive. The rides....a VERY impressive collection with a few world class coasters. (Raging Bull, Viper...) The food caught me off guard as being better than your average theme park. The staff was very nice and friendly, and the ride-ops did an excellent job. 2 other visits last year showed the same high quality of service, staff, etc.
Paramount's Great America was an entirely different story. I was really disappointed by their rides. Top Gun was good, but no stand-out. Stealth only operated 1/2 out of 2 days I visited there. (When it ran it had an OBSCENE wait and capacity). Every other coaster was OK at best. Nothing to write home about. Now, I can have a GREAT time riding rides that aren't world class. (Not everything is a Ghostrider or a Magnum) However, the experience I had at PGA was TERRIBLE. One of the worst examples of how to run a theme park I've ever seen. The staff were unfriendly, unefficient, and often just plain rude. The ride-ops were also doing a terrible job at trying to feed the large lines through rides, and I never saw anything running at near the capacity it could, which is not acceptable when there are huge lines. I could give 12 examples of how poor things were, but I'll just share one story: At 6:45 I went to get dinner. After standing in a line for 20 minutes a food employee told me that the line I was in was closing, and I would have to go to another line. The other lines were longer than the one I had entered 20 minutes ago! I was furious! I just gave up and waited until 10 and ate outside of the park.
A second visit was almost just as poor. I was VERY dissapointed, and it was a shame. I was looking forward to PGA a lot. If they ever write a book about how not to run a theme park, I saw a LOT of things there that should be put in that book.
I hope everyone has a chance to visit Six Flags Great America. It was VERY good!
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- Peabody