I use it in making wine all the time, but I've never tried it with beer. I usually use whirlflock during the last 10 mins of the boil.
When I use it in wine, for 1 gal batches, I use 1 tsp of bentonite and make a slurry with a little wine, maybe 1/2 cup. Whip it up really good, let it sit, whip it some more, let it sit, whip it some more, and then mix it in really good with the rest of the wine. Within 12-24 hours it's so clear that it doesn't even look like the same wine anymore.
I don't really know how much you should use for beer, but I would think no more than 2-3 tsp for a 5-6 gal batch. A little goes a long way, and you don't want to strip out too much from your beer. Wine HAS to be clear, beer doesn't.
If you try it, let us know how it turns out.
Cheers!