I avoid cabs like the plague. Coincidentally, it is also the place you may be most likely to contract plague. No, but seriously. It's expensive. It's usually uncomfortable. It can be avoided with a little planning ahead.
It's not that I avoid them really, but I do see them as a luxury. If you ask me, I can't tell you how much a cab costs, if it's very expensive or not, but I imagine that it's way more expensive than the bus or the subway. I think that people that own a car usually don't have that need, I think the last time I was on a cab it was to drive me to the airport.