I think my initial reaction is for my defenses to come up (just trying to be honest here).
Luckily, for me, I'm the kind of person who questions my own opinions and actions if there's a question that maybe something I've said/done/am doing/not doing isn't as it should be.
It's not even a matter of what someone else thinks of me so much as it is my own personal inner code of ethics. I always try to do the right thing and/or do my best at something.
If someone suggests I might not be, I pick the matter apart and examine it closely to see if the criticism has merit. If (in my own opinion) it has merit, I set about finding a remedy.
If I'm unsure the criticism has merit, I generally keep it in the back of my mind as something to watch for, allowing for the possibility that the criticism is valid.
Finally, even if I think it has no merit, I still keep it in mind, but I examine who offered the criticism and the possible motive behind it. This is such a broad area, as criticism can be found in ALL areas of a person's life.
For a more specific example, I will use work. If my boss suggests I've done something "wrong" my first reaction is to explain why I did something the way I did it. If he still thinks I didn't handle something to his liking, I ask him how he feels I SHOULD have handled it.
At that point, he generally just shuts up and goes away because he realizes I handled it in the best possible way.
At work, about every three months, I actually ASK for feedback so I can improve. Once in a while something is suggested and we try to come up with a good solution.
Sometimes, criticism can be an awesome tool for improving MANY aspects of your life.
Ok, I'm rambling....
To answer the question in a simple way, I take criticism as an opportunity to learn.
Disclaimer: that doesn't mean that I wont object to the tone of the delivery of the criticism and the content if it's delivered in a malicious manner.