Design patterns are a great tool for software developers. The classic text by the "Gang of Four", Design Patterns is a great starting resource. I honestly will have to admit that I don't have all of the patterns memorized, but I do refer to them on a regular basis. Whenever I find myself struggling with a design decision, I flip open the book try to get some inspiration. I don't always find a magic answer, but it can help me straighten out my thoughts.
Design patterns are great, but not anything new or earth shattering. I believe that most good software engineers tend to use the generally ideas anyways. They may not match exactly what is described in the traditional design patterns, but generally they are similar ideas. The introduction of formal design patterns just helped to formalize and standardize these good design principals.
In my opinion, design patterns are not the answer to all things. They provide a framework and common vocabulary for organizing my thoughts, and having discussions with other developers. Since the original book came out others have come up with additional patterns, which in most cases provide similar value. I recommend to all software developers to make sure that they are familiar with design patterns, and have a resource available to use when they need it.