I do it every day. I think it's important to get away from the office, and since I can, I do.
That way I
a) Save money on lunch by eating at home.
b) Get some excersize at lunch which is hardly a bad thing either overall or in the middle of the day.
c) Double my overall milage
d) Get to relax in a familier and comfortable environment.

I know a lot of people who regularly work through lunch and/or eat at their desks. I holler at them all the time. This is not productive in a global sense! We can not concentrate on tasks for 8 hours straight. We need to step away and do something entirely different for a bit - even a short while to reboot our brains and bodies. Little rest, little excersize, little food, little thinking about anything but the task at hand.

Try it, I guarentee you'll be more productive at the end of the week!