Let me just say, it is EXCELLENT. The original anime series was a classic on its own, but I feel Brotherhood has the potential to be so much better. There was a lot of confusion as to what Brotherhood was, so I'll just clear that up first. Brotherhood is a faithful adaptation of the Full Metal Alchemist manga, meaning it follows the manga's story closely. It focuses more deeply on each of the characters and has a much more involving and deep plot than the first anime series, which cut its relation to the manga about half way into its story and retconned everything.
The quality of its animation is, just like the first anime, superb. Action scenes and the like are so fluid; they're really a spectacle to watch. But more than anything else, the plot and its characters are fleshed out in great detail. Edward and Alphonse Elric's journey to recover their bodies is given a whole new light; with talk about "the door" and how their bodies are waiting for them on the other side. Other characters like Scar, Mustang, the Fuhrer, and Kimbley are more interesting than ever.
Though, the greatest difference comes in the form of the homunculi and their leader, who bares a strange resemblance to the Elric brothers' father, Hohenheim. Hohenheim's backstory is the heart of the plot of Brotherhood and is much more interesting and epic than the first anime's alternate plot. I won't reveal anything further, but it is really quite interesting.
All in all, its really quite good and I recommend you check it out. They have full episodes on Hulu, though most anime fan sites release current episodes a lot quicker.