I've used gelatin once, and I did a lot of searching around and reading before trying it. The general method for use is to dissolve it in a cup or two of hot water and add it to the beer IN THE SECONDARY two to three days before bottling. You do not add it to the boil.
Thinking back on that batch (just 3 months ago), I would say that the use of gelatin did little to nothing for the clarity of my beer. I had no chill-haze in that batch, but I also did not use any grains in that batch and THAT'S what actually helped the clarity... not the gelatin. At least, this is what I think to be true.
I decided that the gelatin was just one more step to do for the beer and was of dubious value. I probably will never use it again, unless I want to prove to myself that it was that lack of specialty grains in that one batch that made it come out clear and not the gelatin.
I should also note that I had to work extra hard to clean out my carboy after using the gelatin. The stuff was pretty clingy and I had to scrub more than normal to get it off the glass, even after a soak in VERY hot water.