Over the years, I've played in a few games where cannabis has been consumed by some number of players. I've played in a few (including a game I used to host) where it was openly passed around at the table, and others where it was hastily consumed outside.
Its effects on the game are a mixed bag. Kinda like alcohol. It varies substantially from person to person.
Some people can handle it and still play competently (i.e., without holding up the game, not necessarily playing skillfully), and some can't. You can usually tell the people who can handle it, because they tend to be regular users who bring their own bud. When they come back from smoking, maybe they're a little mellower, but they keep up with the game and don't act stupid. Long-term use means minimal effects of intoxication; it's more like a mood stabilizer for regular users. People like this generally don't create any hassles.
The folks who are likely to create hassles are the ones who rarely smoke weed, like only at parties or whatever (and they consider your poker game a sort of party). Tony in seat 3 went out with the regular pothead to smoke a little, and now he keeps messing up his bets and has to be constantly reminded it's his action. Bonus points if he spills a drink on the table or knocks someone's stack over by accident. For the table of players who have to keep waiting for him to fumble through his turns, the only upside is that his skill level has probably been knocked down a peg or two. But overall, he's a burden to the game and is pissing people off, just like if he'd gotten sloppy drunk instead of stoned.
One major difference between cannabis and alcohol users: cannabis users will very rarely get aggressive about anything when they're high, unless they're inherently aggressive people in the first place. One of the more noteworthy effects of cannabis is that it tends to make people more relaxed and passive (keep this in mind while playing!). Booze, on the other hand ... well, I'm sure we all have a story or five about someone getting completely out of control due to alcohol.
