Parthenon sets the bar for the gyro experience. They are the "Mona Lisa" of Madison gyros. They are the ones that all other gyros will be measured against - and likely fall short of.
The standard gyro/fry/soda order is more than enough for a meal. The meat is plentiful as are the toppings. The fries are done well and are handy for mopping up what's left behind form the gyro itself. Parthenon offers a delicious and consistent experience.
If you want more, you can get the gyro plate, but you'll want to tack on an extra pita or two.
Go to the rooftop on a nice day to enjoy your meal or park yourself somewhere along the street to people-watch while you dine (Springtime is a great time downtown...)
For those of us on the perifery, it's not hella-convenient to head downtown for lunch. But, if you can get yourself down there, it's worth the trip.