I just made an interview with the guys behind
RobotReplay.
If you don't know RobotReplay, well in a few words, it's the most convenient accessibility and usability workshop in the world, for free, anytime, anywhere.
Basically, RobotReplay records all the sessions of your visitors, and shows you what every visitor did on your websites, from scrolling, clicks, completing of forms and selecting text.
The most important thing is that the end result / report is NOT a video but the page itself, and a little bit of javascript, so the bandwidth you save is from 1000% to 5000% perhaps. All you have to do to start monitoring your users, is to put a simple one line of javascript in every page of your website.
Go ahead and do a test run on your website.