To be honest, it depends. In terms of both Hellsing Ultimate and FMA: Brotherhood, i wouldn't exactly call them revamps. They both do exactly the same in that they both go and focus on the original material(their mangas) instead of go off on its own tangents like the original series for both did. So in many respects, its more of a proper telling of each story. So if you where to watch both of them, you'd notice that you quite obviously not watching the same thing twice, and in my opinion, its far better following the original stuff than coming up with a filler storyline.
In terms of stuff like GitS, and quite obviously, EVA, i can't say. GitS i think has only had a revamp in CGi and some scenes, but EVA is a whole new retelling again of the series. In terms of what i've seen mentioned around, Eva is getting nothing bar praise again, as for the GitS revamp, i dont hear much on it. If i look at them, i guess you could say, it could easily go very well, or very wrong. Which also reminds me, i've been meaning to check out the revamp of Mazinger.
