Starbucks has been very successful in New York City for one primary reason. It's convenient. Now, I for one hate Starbucks coffee. It tastes like ass. Additionally, it is wayyyyyy overpriced. What it does offer however is a warm place to wait at when you're meeting up with people. In a place that is so dependent on mass transit, having a place to simply sit down and wait for others is important. Now are there other great coffee places around? There certainly is, but very few of these places have the means to seat large amounts of people or provide things like free wifi.
Now see, this is an argument for Starbucks that I can get on-board with. I will agree that most of their locations in large American cities are fairly comfortable and provide a place where you don't have a waitress wanting to kick you out so that she can turn the table. I lived in Atlanta for 15 years, and although we drive a lot more than you, the argument still holds - you can sit there for quite a while, laptop and all, and no one will boot you out if you don't spend more money. *nods*
But yeah...the tradeoff for that is that you have to buy a $5 cup of coffee that tastes like ass.

Here in Anchorage, it's a much smaller and more casual city with lots of places you can sit for hours without anyone booting you out. But because it is so damned cold here, we take our coffee a lot more seriously. Which is why the other coffee shops (mainly Kaladi Brothers, but a couple of others too) have held their ground very well against Starbucks.