With my tap water I could only brew good stouts and porters. I have had to do more to brew good German Pilsners. The water stuff has paid off. If you are lucky you might be able to brew good Amber to Brown beers with your water, and that might be what you like to brew!
+1 to that, and to Martin's comments as well. I can make anything darker than a Pale Ale with my water, but for Pilsners, Kolsch, Cream Ale, etc I had a heck of time getting something I like. It was only after I started messing with building my own water in the last year that I am getting results I like. Other people have the reverse problem (they need to mod their water for dark beers.)
In either case, learn about your water and tweak it if you are serious about accurately brewing a wide range of styles. This definitely involves getting a pH meter and learning how to use it, and for what. And I cannot understate how helpful both Martin's and Kai's spreadsheets have been in learning to tweak/build water. Various other people on this forum have also been extremely helpful in educating me in this field. (big props to "HopfenUndMalz"!)