I liked them a lot in 2. They just felt disappointing to fight in 3 and Reach. Especially 3. I hated the "turn of events" that led to Elites basically becoming allies. I loved fighting elites a lot, Brutes were fun to fight to but I felt they fit in well in the rather infrequent doses they were put in.
I couldn't imagine how they could make Brutes replace Elites. But they did so by basically making them weaker. It's cool that they had ranks now, and they were more numerous and worked together as squads, but it still wasn't a good replacement for elites. Although I did like the Brute Chieftains and their hammers and their crazy fucking 5 story thundering leap of the gods. That was awesome but those thrills were few and far between.
The most relevant thing I guess you can say about the Brutes was their ability to berserk. Which made them a huge problem in 2, their speed almost triples and even in the Heroic difficulty, all it took was one shoulder tackle to smear the chief across the floor. Yet in 3 and even in Reach, their berserk barely makes them more threatening. I just thought they were very disappointing.
In Reach, they were.... alright. Still not what I expected them to be but a bit closer to that. They were still mostly easy to kill, but the higher ranking Brutes were much more significant threats. With their overshields, armor plating, and armor lock. The one sad thing about them is their Hammer Brute doesn't seem to use that Mighty Thor sky crushing hammer leap anywhere near as much. But they do have a whirlwind attack. In Firefight I almost always splattered by that damn attack.
*I frequent the firefight mode in Reach alot, and with the catch skull on, I noticed that Brutes are insane with grenades. I dunno what it is, but they have almost 100% accuracy.