First tankful of gas in my new '09 FWD Enclave was 24.0 mpg. That was all highway, but averaging 70-78 mph. Now, with 900 miles on it, I'm getting around 21 mpg overall with 1/2 city, 1/2 country driving. If it's like any other cars I've had, highway mileage will improve from 24 mpg.
Not bad for this large a car!! It's the quietest, best engineered car I've ever had. Buy a Honda if you want, but it won't be anything like the Enclave. Buick did their homework and designed the quietest SUV as well as one of the best-looking.