There's a lengthy, cautious and sensible sounding article over at Nature News about the giant experiment in public health that cannabis legalisation is going to represent.
There are so many complicating factors when trying to judge what may happen (or even in working out which other countries' present experience make for a good comparison) that prediction seems little more than guesswork.
Still, I lean towards the "it'll all end in tears" side, as you may expect.
Update: as I have noted before, it's actually pretty astoundingly weird how drug problems differ from country to country. Russia has virtually always been off its face on alcohol; China has had its opium and now meth and heroin problems on quite a vast scale; I'm not sure for how long Japan has been drinking heavily, yet they barely touch anything else (apart from tobacco); apparently some small Pacific Islands are actually way at the top of the table of heavy marijuana users (beating the Caribbean, surprisingly); Sweden, while famously relaxed about sex, is an outstanding drug free country, although their controlled use of alcohol is no doubt partly due to a system (a State monopoly on the sale of any above 3.5%) which would horrify a libertarian; and who would have thought 20 years ago that ice would become a chronic problem in rural Australia, more so than in the inner cities, it seems? (As it happens, I was today talking to someone from Western Queensland whose family had been devastated by it.)
My point being - given the curious lack of any clear pattern about which country develops overuse problems with which drugs, it wouldn't be surprising if full legalisation of cannabis in one nation did not lead to any great problem, while in another place it sent the country into a sort of stoner lead economic decline.
It may sound like I'm just giving myself an "out" if, in 10 years time, everyone declares cannabis legalisation in the States a great success. But honestly, I think I am making a valid point.