I had TIffen glass grad filters years ago. For film, I think they did have a cast, but much, much less than the Coken they replaced. I've replaced the TIffens since with Signh Ray grads and they are dead-on neutral. However, with the dynamic range of today's sensors and using a little bit of post, I seldom even think of bringing my grads in the field today. Even with longer lenses, the Signh Ray didn't seem to create focus or sharpness issues.
Today, I can create a higher quality image by usually shooting additional exposures in the field and then crafting the image in post. For me, this gets me around the limitations of grad filters. Sometimes, I use multiple exposures, sometimes simply double-processing the file. In any case, the thing that was always an issue to me is that the filters worked great with an image with a straight horizon, but if there was anything that stuck above this line, it darkened whether I wanted it or not, such as trees, rocks, buildings, canyon walls, etc.
For some images, the grads are fine, but for most of my work, post is a much better approach.
