With the Vari-N-Duo, you will get some unpleanant effects (like uneven lighting) at wider angles. It goes away around 24mm from my experience. Sing-Ray says that you should lower the ND setting if this happens. As for vignetting, yes, you will get some at the wide end of your zoom, but this is true of any filter that's not oversized for the lens you are using.
Despite these two shortfalls, I love it. Being in Hawaii, there is sometimes a little too much sun. However, I shot an image last weekend under a bright sunny sky, and I pulled off a 10-second exposure at ISO 200 with my Vari-N-Duo. It's a great creative tool.