Fun for the player, annoying for everyone else. My main Shep in the SP is a vanguard so I know the class well and enjoy it, but in MP it's a different ball-game and without being able to pause the action a vanguard is a liability both to himself and the other players. For one thing, he gets into trouble that he can't get out of and for another, he sends all the enemies sprawling throughout the map instead of concentrated in one kill-zone that ranged players can pick-off.
Whenever I get a vanguard in MP I groan because it means my infiltrator will constantly have to run around in order just to find targets and even when he does find them, the vanguard will be forcing them to move around so much that they're hard to target. Whenever I'm in a an MP without a vanguard, I do very well and often top at least 25 head shots and either manage middle to highest depending on the skill level of the players I'm with (I don't put myself at the top of the skill list, just middling). With vanguard I generally manage only about five and almost always get the lowest score.
What's worse is that the very vanguard that's spoiling everyone else's shots not only gets flat-lined during at least half the waves, but they also generally only get slightly better a score than I do, meaning all that chaos they're causing results in jack contribution to the success of the mission.