I absolutely love them, I tend to colonize low and rely heavily on my economy. By the time I build my first dreadnought or death knight (which is usually my first attack ship) I've gotten atleast to 4D phasing or simmilarly up the research spire for Excal tech. In fact, I usually skip Overlord tech and just shoot for Excal anyway, not to mention the nice affects alot of the nessicary techs give along the way. It is pretty close to the top of the tree, but you can get there and have a large ammount of unresearched tech, so if you really try you can get there fast.
As for how useful they are, well, they do cost alot, but by the time you start putting them out you'll love them. When one can clear out a sector a turn, just imagine how pleased I was last game with over 50 (I turn research all but off when I get to final frontier).
P.S. a bit off topic, but are there bonuses to different victories? I am under the impression that military victory scores higher, I hope I'm not wrong