Never finished the original FMA show. The at times sluggish pacing didn't really do it for but more importantly, the show got very depressing and angsty at times. I don't really dig that, which explains my mild dislike of NGE, because otherwise I quite liked the style of that show.
Brotherhood was more upbeat I thought, or at least didn't dwell too much on the tragic stuff, of which there was still a lot of. Reason for that is the accelerated pacing I guess. I didn't have trouble following things, though the fact that I watched the original show at least halfways helped in that I suppose.