I'm just starting out with the multiplayer in Starcraft 2. Is it better to play against the AI or suck it up and start playing actual humans to learn how to play? I've watched vids and replays and played a few games against both AI and humans.
Regarding AI vs Real Humans, you'll never get better experience than playing against human beings. That being said, the AI can be used as a tool to gauge your Build Order, dealing with timing attacks and executing counters.
But your ability to shut-down the AI is not a good way to compare yourself to other players. So being able to defeat Very Hard doesn't mean you're about gold level play.
If I had to break my playing against AI vs against Players, I would say about 10% is AI and the rest is playing against real players. And the replays against humans are the ones I really pay attention too because those will teach you the most.
Also try the "expert" level single-player challenges, if you haven't already; they are designed to teach you some skills that will be handy in multiplayer matches.
Play against humans. It's not that the AI isn't good, but they're "good" in different ways.
They don't prepare you for defending against "cheese", and they usually just try to come in through the front (where your defenses usually are) instead of going around through drops or air attacks.
Definitely play against humans. Personal learning style is a factor of course. I myself can't really be bothered to spend hours practicing the same surround or whatever over and over again until I get it exactly right. I'll just jump in and try it on a real player.
What I've learned most from, really, is getting owned by some player and then googling/youtubeing his strategy to see where I went wrong in my counter. Sometimes I'd find out I countered correctly but my macro was seriously lacking, etc.