We only have the most basic of cable ($9/just networks) and rely primarily on Netflix. I've used Netflix streaming on the Wii, on our PS3, on my laptop and on our iPad and have never had a problem. Speed and quality are definitely affected by your internet connection. The quality is fine, it's definitely not HD, but that doesn't bother me at all.
The selection on brand new movies isn't great, but easily supplemented our actual DVD queue. The streaming television show, documentary and indie film selection is surprisingly pretty good.
As for Hulu, I've never had any luck, I think it's a craptacular service. However, they are rolling out a pay service that is supposed to be better.