It's a fantastic ability when used for setting up/setting off biotic explosions.
Let's take a Drell Adept in multiplayer for example:
Lesser enemies with no shields or shields down: Use Pull to render them helpless, them Reave to detonate.
Larger enemies/bosses: use Reave on them, them throw some cluster grenades to detonate.
Biotic detonations are extremely powerful and unlike some abilities like pull where it works only on unshielded targets, you can throw a Reave on anything, including shields, barriers and armour.
Lots of abilities detonates Reave: warp, throw, cluster grenades, shockwave, biotic sphere, charge and lift & slam in single player.
In multiplayer, if you see other biotics, you can help the team by basically spamming Reave on everything. Assuming your team mates knows what to do, they should be detonating after your reave. Makes short work of everything.
My reply is only specific to Reave, but there are lots of other combos that you can pull off.
Relix28 created a very comprehensive list of all possible combos here: http://social.bioware.com/forum/1/topic/343/index/9629730
Just came out from a gold game with someone spamming reave and another spamming overload. I was an Asari adept and spammed throw, an ability that sets off both biotic detonations with reave and tech bursts with overload. Absolute fun, fireworks and destruction. =)