I live in Connecticut, home of Foxwoods and Mohegan Sun. I've never actually been to either. (I'm not a gambler.) They are both gigantic complexes (Foxwoods especially) and people come by the busload from all over the Northeast.
From what I've seen, it's good for the tribes, and it's good because it creates jobs for both Native Americans and non-Native Americans in the area. I don't think betting on horses is such a huge revenue generator around here... but like I said, I'm not a gambler, so I'm not sure.
If anything, they're stealing revenue from Atlantic City... and who cares about Jersey anyway? j/k