I released the full version of my app back in January for $.99 a few weeks ago I released a Lite version (ad supported) of my app. As of now I'm not receiving any substantial revenue from ads in my Lite version, if anything it's sort of hurting sales and if it doesn't turn around I may remove it at end of the month. This is just my personal experience and it may be different for full free versions supported by ads compared to that of Lite version. Also it will deferentially vary from one app to another and the type of app it is.
The app I'm planning, after I finish my current one, I'm going to experiment with making it a full free version with ad support and see how that goes.