Most of the time, when we think of Italian restaurants, we're thinking of Sicilian cooking. Red sauces, spaghetti and meatballs type dining on red checkered tablecloths, mafia men in the shadowy corners.
This is not such a place. Mario's is an elegant restaurant (which is reflected in the steep prices) with a fabulous wait staff, serving authentic northern Italian cooking. This is the Tuscan-style meal -- seafood and heartier dishes, not as much tomato sauce or pasta, and more arborio rice. Paella and crusty breads are more popular here than pizza.
Mario's is in the process of moving. The locatin they are in now -- way out near the industrial district -- is a beautiful building in the wrong location. They've purchased a place just across from the Mission Inn, which will allow them more foot traffic and incidental stop-ins, much more accessible to most of Riverside.
They recently added a lunch menu (previously only open for dinner) on Fridays and Saturdays, which may move to an all-week affair when they move. The prices, as mentioned, are steep -- it will easily cost you $40 for two persons without wine inclluded, but is also easily worth that price. Whoever they have in the kitchen is an artist as well as a chef, and deserves all the credit he's been given in recent years. (Mario's has won several dining awars in this area.)
You probably shoulldn't just "decide" to go to this restaurant. Calling ahead for reservations is recommended on weeknights, pretty much required to get a table on the weekends. If not, you'll find yourself with a very long wait.
I will warn you now: once you've eaten here, it will spoil you for other, less quality Italian restuarants. Tomato sauce will never be the same.