After my not-so-pleasant experience the cheap Cokin filters, I thought that I might as well make the investment and get the best of the best filter system used by the best of the best photographers.
As you can see in the photos below, using one of the filters introduces a small amount of vignette. I believe this actually adds impact to the shot. The sharpness of the photo has not been affected what-so-ever as well as the colour (unless the white balance automatically changed in camera). For the shot with the big stopper, I decreased the ISO so that I could use a slower shutter speed. This was so I could see if using a longer exposure would bring out any glare. I couldn't see any signs of light leakage, but I could see, what looks to be, a mark caused by dust on the glass elements. Meaning that it's worth noting the importance of keeping all glass elements clean and dust free.
Big Stopper (with a small exposure increase in photoshop camera raw):