As has been discussed before this would be terrible for anyone who smokes pot. Quality would go down, price would go up.
Many theorize that illegal activity would increase as well because the government wouldn't want anyone encroaching on their business.
Lewis Black said it best last night on the Daily Show:
http://watch.thecomedynetwork.ca/the...es/#clip147733
(relevant part starts at about 1:57 of the video)