Its a bit of a stupid concept as you may mark one as a best answer and then a much better answer comes along. What do you do then? give another "best answer" accolade? I preferred the days of one star, two stars or three stars (if I remember rightly).
If you click a different best answer your best answer switches to the newest one. We've not got it implemented on the site, but we do record them (and when implemented the best answers will "pop" into view somehow)
I still feel that stars were best AB spare Ed. That way you were kind of reassured that someone had read your answer and no need for the poster to keep saying thank you as extra answers appeared. Lately out of each ten answers I post about 2 get an acknowledgement/thanks.