I had an 04 M6 GTO for about a year. Used it as my daily. The get almost 30 mpg with lean cruise enabled on a flat highway going 60mph without a head wind. Add wind, hills, or increase speed, and MPG goes down very quickly.
The interiors are very comfy, but if you intend on putting a kid in the back seat, you will hate it. The seats move very slowly.
Insurance wasn't much more expensive than it is on my 04 Comp G that I have now.
Overall, it's a fun car. But if you are using it as a family car, I would suggest that you look into a G8.
