I strongly recommend you stick with Vanguard at least until you max out charge and upgrade your shotgun. At first I hated it, but since I've maxed it out and figured out how to use it properly it the most enjoyable ability I've ever used in a shooter. By midgame, maxed out charge with upgraded shotgun will let you take down normal dudes with just half biotic barrier and health in about 2 seconds, in slow motion, causing the dude to fly across the room while you fill him full of buckshot, and you can yell expletives while doing it, and it moves you to another part of the map. The key for me was figuring out that it's suicidal to use on a guy in midfield with three other guys shooting from across the room, but awesome if it's two guys in a flanking position behind cover and you are simultaneously blowing up the other dude with a maxed out tech ability.
So, long story short, it's awesome if:
-- you can kill the guy you are using it on with your shotgun in a couple of seconds
-- that guy is behind cover or there is no one else on the map
-- you are simultaneously killing or incapacitating any other dudes next to the guy you are charging with your allies' abilities
and suicidal in every other situation.
It's also slightly bugged in that it will only work occasionally on a guy in melee, but shockwave is good for that so no biggy.
Another fun thing you can do with charge to take advantage of the temporary double strength shield you get once you upgrade it is charge a land based boss, shoot him a couple times with shotgun and then run away to cover before he clobbers you. This sometimes gets you killed but when it works, it's hilarious.