Reave is awesome! Don't bother using it during the Collector Ship though. Most fights with the Collectors - they stay put, taking cover - only the possessed one moves toward you. Then it's great to use. But on the Collector Ship you are in such a small area that you need to take out those Collectors as quickly as you can before they move up on you. Best way is not to not worry about your team - make sure in the options they are using their best ammo/bio/tech abilities - just focus on taking down the collectors (really try to get as many drones as you can before you worry about the possessed one) Hopefully your team will keep him busy. Best way is adrenaline over and over with inferno ammo and the assault rifle. Surprisingly I liked the first assault rifle you get the best. You can do a steady stream of bullets until the clip runs out. It was easier for me to use than the Battle Rifle.
Also when you are on the suicide mission and need to open the vents best to not worry about reave then too. You got limited time to open the next vent and adrenaline and the assault rifle takes down the Collectors the quickest. A sniper headshot with inferno ammo on the drones will take them out in one shot - but you can lose too much time reloading. Good in situations where you have a lot of ground to take cover and there are no time restraints (like Horizon and parts of the suicide mission). Though if you are an ace with the sniper - that might be a good way to go now that I think about it.
Besides those two situations, I used Reave A LOT! It helped me make it through a lot of tough situations.
Good luck!