Gus's is the kind of place you go to for the experience rather than the food, and certainly not for the location if you live in downtown Madison like I do!
Gus's is a perfectly adequate greasy-spoon diner that's well outside of Sun Prairie's "downtown." The decor is reminiscent of a classic '50s diner (or at least, it does a passable imitation for those of us too young to remember the actual '50s), replete with jukeboxes at every table and a red-white-and-chrome color scheme. The waitstaff are decently friendly and competent. The menu, which includes a fun "In the '50s..." list of ZOMG EVERYTHING WAS SO CHEAP shockers, is huge and very unhealthy. I greatly dislike the processed taste of the creamy soups, but the sandwiches, omelets and burgers are all tasty and beyond filling. Avoid the chicken tenders -- a friend ate them once and they were inedibly salty. The same friend pronounced the coleslaw "meh" (I assure you, he's a connoisseur -- I myself don't touch the stuff). No one I've ever been there with has ever sampled the desserts. If you choose to assuage your nagging conscience with the salad bar, you'll probably be a lot more disappointed than if you'd just gone for the Denver omelet... mmm. Overall, it's the kind of food that hits the spot when you wake up late on a weekend day and realize you are starving.
I'd probably have given it 4 stars, but the coffee kinda sucks.