Much like others here, I've found Greg's to be my favorite store around for comics. It's loaded with back issues and the staff is always extremely helpful. When asking about a specific comic, you don't get a "point" in the proper direction -- they walk over to the section and hunt down your comic with you and if they don't have it, they offer alternative solutions for you.
On top of the great service, they also have great prices. Most of the time they're offering lower-than-cover-price deals, which makes buying here a real treat.
To me, Greg's has a real incentive to make me want to drive the extra distance past some other closer comic shops.