I have used them in the past, but not lately. I've found it very useful, especially when I was mentally distracted or had a hard time focusing, and kept reading the same line, over and over.
It's similar theory to reading in narrow columns. I read faster, and am more focused, when I read in narrow columns. For me, it seems to create a mental rhythm.
Here's the one I've used online.
http://www.spreeder.com/