I used to get some income from it, wasn't much though - £10 to £40 a month.
However, Google Adsense changed their policy's and decided that my site didn't comply with the new policy, so they withdrew.
I would not suggest it is used as the only revenue method. It can be rather variable.
Make sure you read all of the Google policy's in particular Google Webmaster Guidelines.
What sort of site are you planning / do you have setup already?