I think most people start by going nuts on free books. That's what I did, then I got tired of the mediocre quality of many of them. I don't believe I have changed my buying habits, or rather, I now may buy less than before, because I still read a lot of free books, I just don't read any free book I can get my hands on. And for pbooks I tend to use my local library more, so that I don't get weighed down with another paper brick
A long time before I even new about ebooks, I discovered that if I bought too many books at a time, there was a bigger chance I would never read some of them. So I never buy more than 2-3 books at the same time, whether paper or electronic. And with ebooks, I don't even need to buy them in advance, since I can get online and buy or get a new book any time I feel like it.
So, bottom line is, I read more but I think I buy less than I used to.