I teach using beamer slides. Our lecture rooms have a thing called Sympodium (from smarttech.com) which means you can write on the monitor with a kind of stylus and annotations appear on top of your slides. I find this occasionally useful for correcting typos but generally I don't like it. The writing is always large and untidy. If you have beamer slides that display in several steps, then the annotations disappear when you move to the next step.

If you want to display handwriting through a projector, you can write with real ink on real paper under a document camera/visualiser. I find that that works much better than any purely electronic system that I have seen.

I know that a lot of people have a strong preference for blackboards instead of slides, and I agree that some of the reasons they give are valid. However, I personally find it much easier to keep everything well organised with slides.