There's nothing wrong with a Lenten fast. I don't know that there's also anything wrong with taking the 40 days of Lent and using them to kick a habit.
Our time period of 40 days is intended to mirror Christ's own 40 days in the desert; where he fasted and prayed. He was also confronted by the devil, who of course tried his best to distract Christ from that holy time.
Bibilically, the purpose of the fast is not to kick a habit; the purpose is to take things out of your life in order to make room for God. If I'm taking a 24 hour fast from food, for instance, my job during that time, when I'm hungry, is to be praying to God for illumination. Prayer and fasting go hand in hand.
I, myself, have decided to take a different tack with my Lenten discipline. I'm choosing to write. To take the time I would spend watching TV or checking out Facebook, and get some thoughts out of my own head. I've decided to realize a dream, that I feel called to. As Jon Acuff tells us in his book Quitter, what good is a dream that's only in your head? So I'm going to write during Lent, with the hopes that I can continue it after the fact.
If you are taking the time to give something up during Lent as a discipline, I would ask two questions:
1) Am I giving this up to make more room for Christ in my life?
2) Am I going to go back to it after Lent? Or is this not just going to be a Lenten discipline or a Life discipline?
When we think of dieting, we more often think of what we're cutting out of our diet. But what we eat overall is in fact our personal diet. A diet is also what you take in. If you're going to give something up for Lent, make sure as you're doing that, that you're taking in more of Christ.