I have a 2005 G6 GT with the 3.5 liter V6 and I recently got 35 mpg on a highway roadtrip. That's the best mileage I've ever gotten in this car, it has 40k miles on it, and I was pretty happy with that. It was mostly flat high desert straightaways with some rolling hills and small mountains every once in a while.
In city driving I usually get 21 mpg on the dot. Which is not so great, but certainly could be worse.
For a v6, I'm pretty happy with it and any money I spent on a hybrid would not make a large enough difference to cover the initial cost of the new car. I had been thinking of upgrading to a Prius but I will probably drive this car into the ground and then buy a small electric or hybrid in 2012 when Toyota and Honda come out with their latest hybrid technologies.
There have been a number of electrical gremlins with the G6, and the dealer experience has not been positive. Which is why I will probably get a Honda next.