It all depends. I am lucky enough to have a REALLY great library system here. My library is small and doesn't have a ton of what I like to read, but they are linked to a bunch of libraries statewide so I can easily browse online and request books through inter-library loan. I usually limit myself to about 3 books at a time, one is usually a slower non-fiction and then others that I like. We have a 3 week checkout time on most books so I rarely go over (unless I forget, but they send me handy reminder emails) and the over due fees are small. Like .20 a day or something.
I love books but buying isn't quite conducive with my grad school/unemployed/substitute teacher lifestyle as of late. Lately I've only bought at library sales (I love getting old books from these!). If there is a book I'm dying to read and can't wait for it or have an excellent coupon I'll go buy it. I use GoodReads
to help me keep track of my read books, books I want, and to find new ones. I have a few friends on there so I can see what they are reading as well. I also have the iPod/iPhone app to take with me so I can check wherever I am (as long as there's wifi since mine's just on my iPod).