When I was a kid, I loved it. My nephews (5 and 3) went on it in May. The 3 year old didn't care one way or the other except about the dark, which he didn't like. The 5 year old was annoyed that it wasn't the Tower of Terror.
My friend will absolutely not take her children (5 and 2) on it until they're older. She believes it will totally freak them out, especially the 5 year old.
So it depends on the kids, and you know your kids best. You could try them out on Pirates first, there's some darkness and scary stuff there. If they don't like Pirates the HM is probably a no go. My nephews were mildly interested in Pirates.