You get exclusive items for Mass Effect 3 if you play the demo of Kingdoms of Amalur: Reckoning, but that shouldn't be the only reason you play it. From the demo, Reckoning looks like it could be a fairly entertaining and worth-while RPG. If you played Fable and thought, "Man, this combat is too slow!" then Reckoning deserves your attention.
The demo for Kingdoms of Amalur: Reckoning is available now for download on Xbox Live, Playstation Network and PC. It gives you forty-five minutes to explore the open world of Amalur, which is plenty of time to justify or rule out a future purchase.