Success doesn't seem to have spoiled quarterback Joe Theismann, and the same can be said for his restaurant in Fairfax, which since the Redskins' Super Bowl triumph in January has been expanded and now has a piano bar.

Theismann's, tucked away in the strip of stores known as Baileys Crossroads, isn't just a jock hangout, although it draws heavily from the sporting crowd. It's basically a solid family/neighborhood restaurant that probably would prosper even if the owner were named Joe Smith.

Theismann apparently has profited from the many sad examples of athletes who have tried to cash in on their names only to see their businesses fade along with their fame. What you find at Theismann's -- outside of football season -- is a well-run restaurant where you can have a full-course dinner for from $5 (yes) to $50 and can expect good service whether they know you or not. On game weekends the place is a madhouse, of course.

While this account is based on half a dozen visits to the Fairfax restaurant, spaced over several years, a friend who's also familiar with the Joe Theismann's in Camp Springs, Maryland, says the atmosphere and menus are similar except that "the Camp Springs one is larger, has a salad bar and more entr,ees and a dance band."

The extra added attraction at both places, any off-season evening, is the possibility that your busboy might be Ol' Number Seven himself. His frequent appearances at both restaurants no doubt contribute to the enthusiasm of the employees, who appear to be mainly young, pretty people. "Joe sat down at our table once," a colleague recounted, "and he was just like one of the family." This warmth seems genuine: For all that Joe Theismann has spent much of his life brashly promoting Joe Theismann, he's the sort of fellow who, while out shopping with his family, will notice, and gently offer his autograph to, a young fan who's too shy to ask.

At the Fairfax Theismann's, the pianist -- usually Walter Clark, Raif Jenkins or Jon Carroll -- starts warming up at 9:30 (on Wednesday through Saturday nights). By the time the coffee and cognac have gone around, people are calling for requests and the evening takes its own shape, depending on the crowd. "Hail to the Redskins" isn't played as often as you might expect.

The food portions are generous, the preparation generally skillful but occasionally uneven. Five dollars will get you the house salad, a fine half-pound hamburger with thick-cut French fries and a beverage. The entr,ees top out with steak and lobster tail at $18. To spend $50, you'd have to order salad, soup, appetizer, entr,ee and dessert plus keep the bartender pretty busy. JOE THEISMANN'S/FAIRFAX -- 5912 Leesburg Pike, Baileys Crossroads. Piano bar Wednesday through Saturday. 379-7777. JOE THEISMANN'S/CAMP SPRINGS -- 5859 Allentown Way. Live disco from 9 every night, five dining rooms. 449-7500.