He's not the best defender in the world, although because Matt Phillips skinned him once he gets far more stick than he should do on here. He's perfectly capable of defending against most bottom half teams when we're at home, mainly because their wingers rarely cross the half way line, and offers a better option than Clichy when the opposition do have 10 men in their own box. In that situation the defence is delighted when we pass the ball around the edge of the 18 yard box, 5 and 10 yards at a time, what they don't want is someone like Kolarov scorching crosses into the box for 90 minutes because they know eventually it'll fall to one of our guys who'll stick it away.
It's basically how United play when they're not counter attacking. They just consistently get the ball out wide, usually a Carrick or Scholes diagonal from the half way line, then look to get the ball into the box, not necessarily at head height because whilst Rooney's good in the air, the rest of their attackers aren't really, so they drill it in either on the floor or at knee height and whilst it looks jammy that the ball deflects off three players and ends up leaving someone with a simple finish it's a reasonably tried and tested method. The only problem is this tactic doesn't work against better teams who don't camp in their own box, but we play Clichy against those teams so no worries.
Kolarov's certainly the best reserve left back in the league, and seems like a great guy to have around the squad.