I wish I was better at design. On the other hand there's this wonderful principle called division of labor, and while I might spend 10 hours designing a mediocre website, a good designer will design a great one in half the time. This has actually happened to me. After spending a weekend building out some pages that I thought looked pretty good, my designer friend sent me a photoshop mockup that made me decide to totally scrap what I had made, and go with her design instead.

The most productive I've ever been has been while working in tandem with a good designer, because it frees me up to do what I do best, and that's write code.

Now, I will say that it's important for any web developer to understand principles of design, because understanding what your users actually want is crucial for both backend and frontend work, thinking about user interaction, your data model, etc.