I've taken some great classes and some so-so classes. But I've never taken a class that I didn't get something out of. A whole lot of it has to do with your attitude going into the class. If you're one of those meek, sit at the back and don't ask questions ladies you're probably not ever going to get much out of the classes. I can't tell you how many times I've taken a class with somebody that complains after the class about the whole thing and the main problem was they sat there on their hands and didn't ask questions because "they didn't want to be a bother." The teacher is generally going to think if you're not asking questions, you're not having a problem. If you want the teacher to engage with you then engage with the teacher.
Most classes have handouts before the class so you can see what the project is. You should know before you take the class if you want to make the full class project or only want to learn the pattern, technique, etc. and buy fabric accordingly.