Bladerunner2 -- 90% of my beef is grass fed (and free
). I love it! Some here will dispute the benefits, taste & flavor, etc. I have been working indirectly with a man who is pioneering a "Back to the Basics" beef program and have reaped many wonderful benefits!
Whether or not your beef will cook like ordinary corn-fed, grain-fed beef depends on the amount of back-fat on the animal at harvest time. If the critter was allowed to finish with 1/4 - 3/8 plus back-fat it should cook like any other beef. Less than that and the too lean and too dry issue will come into effect.
If your brisket looks well marbled and has about as much fat as a regular brisket you'll be ok. If not, I would cook it like any other brisket but would consider basting / mopping it a time or two - may even lay a strip or two of fatty bacon on the flat to compensate.
Hope this hepls. Post again if you have other questions.