What fascinates me is that the people who don't care for Haken generally cite the same two reasons. They don't like the singer and they feel like there's pointless wankery. Those are my two biggest problem with DT. Haken to me is a big improvement.
I think it might be fair to say that in Haken, generally, the wankery is more of a "here's some completely different musical style that you didn't expect" sort, while in DT it's generally more of a "here's a bunch of fast solos and some weird time signatures" thing. And then some people would prefer one or the other.
FWIW, between the two bands, I prefer Dream Theater. I think James is a stronger singer, I like that the keyboards have a stronger lead presence (whether it's KM, DS or JR), and I think DT's catalogue captures a greater range of styles and moods that line up with what I see as significant (while it's possible that could change as Haken's discography grows, I would say the same thing if DT's discography only included WDADU/IAW/
Awake/ACOS/FII—four albums and an EP, the same as Haken's pre-
Vector).
But that isn't to sell Haken short at all. I would put
The Mountain and
Aquarius right up among the top 4 or so DT albums. And I would actually disagree that
Aquarius feels derivative of DT. That album has a distinct style all of its own. With
Visions, I think the comparison has some real legitimacy.