Be sure to get a thin metal-rimmed filter. The above images were shot with a metal-rimmed filter. The image below was shot with the plastic-rimmed Raynox filter bundled with the tube. Note the vignetting in the corners. Also, you want to be sure to get a multicoated filter.
Uncoated filters give reflections at the front and back surfaces.
These degrade sharpness, and show up most noticeably when there is a small light source within the image. You will see one or more ghost images from multiply reflected light off the front and back surfaces. A multicoated filter eliminates this problem. Not all multicoated filters are equal. Some have multicoating on only the back surface.