I prefer blackboards/whiteboards to any other option (even OHP) for the following two reasons:
1) I find nowadays (having been giving talks for about 15 years) that I only semi-prepare them now, and never use notes, and just say what comes out: I have a "plan" and talk around it. This way I'm guaranteed to never give the same talk twice, which is interesting for me and interesting for the audience.
2) Can't be bothered to make slides!
