The private prison industry would suffer, not sure how big of a lobby they are (or if they would be effective because --->). Governments at all levels would benefit from lower incarceration costs (so win/win/win). And there's revenue from taxes (win x 4). The only downside I see is that people would just continue using marijuana as they did when it was illegal, and no more threat of legal problems. That's not a downside of course, in fact it is another win, this one for personal liberties (win x 5, this one for the people).
I had the pleasure of visiting and purchasing legal weed in Colorado, we actually went with the mayor of the town who hadn't been there before. Having the mayor with us got us VIP treatment, we skipped the line entirely (they didn't look happy...). Then we went back to his office and... sampled things. It was fantastic.