In the most important freeways (Marginal Tietê, Pinheiros, etc.) it is really bad on rush hours. It is always very busy, but it flows and is mostly organized, so GPS often gives you a correct estimate of arrival, but if there are accidents or some work happening you can forget getting in time.
In other times it may be "ok" and inside the neighborhoods it's fine too.
The buses aren't so good, but the subway is great and it can take you most places, but you should do some research first because it won't reach the whole city.
If you plan on staying mostly on the central area, subway should be good enough for you!
Comparing to other cities, São Paulo traffic is really busy and can be scary, especially because if you miss one street it can cost you a long time and distance. But as I mentioned, it's organized and drivers are often respectful, you just need to be careful about the motorcycles.
Hope it helps!