Generally, I'm not a big fan of modern billboards. These new "big screen TV" style billboards are very distracting to me, especially when driving at night and they have a super bright display on them.
As a kid, however, I did enjoy the Burma Shave road signs, and it seems that billboards back then were pretty creative.
I would probably feel much differently if there were more billboards like these:
http://www.rockinghammemories.net/chevybillboards.html