I've had mixed results with grass-fed beef. Sometimes I can't tell the difference, and sometimes it has a definite barnyard tang. We bought some beef (frozen) at the Ballard farmers market from Thundering Hooves last year. They make a point of feeding their cattle entirely in the pasture -- no grain at all. Apparently, some "grass-fed" beef is fed mostly grain and then "finished" on grass for the last part of its life. It was good, but it wasn't life-changing.
In my experience (and I know that I'm probably slaughtering a couple of sacred cows here ), once you've reached a certain level of quality -- that is to say, once you've purchased a premium piece of beef from a butcher who knows what they're doing -- how you prepare the meat makes a lot more difference in how it tastes than its provenance.
That said, I'll be happy to take the Beef-si Challenge with anyone who wants to throw down the bux for some top-quality steer.