No they're not fixed. The casinos get their money without having to cheat you. Plus, they could lose their license if they do. There are definitely differences in the machines. You have to check the pay schedule for the best ones and play perfect strategy to keep the casino advantage in check.
I like to play the slots, and they are not really fixed. Some times you might get lucky, most of the time you don't. I play a lot when I go to Atlantic City... spend hours just having fun with the 1cent machines!
I think the casino's make more than enough money without risking their livelihoods by fixing the slots. They're great fun though and hardly seem like you're spending as much time/money as you do.