One of my favorite online classes. I had taken a class on compilers and programming languages with Alex Aiken many years ago, but I needed a refresher. My previous course was more intensive, but for my needs, I preferred this course. The instructor, Wes Weimer, was extremely knowledgeable and fun, and the quizzes and exercises kept me on top of things. The class was well thought out and had a number of satisfying problems to solve. The last final exam question in particular beautifully summarized the lessons of the course.
The biggest downside of the class is that there's not enough of it. Although it's advertised as "Building a Web Browser", it's really more about writing a lexer, parser, and interpreter. Those are key to the process, to be sure, but I was hoping to have my own browser at the end, and missed a bit of the big-picture motivation for all the regular expressions, finite state machines, et al. that we covered. I would also have liked more material about the design of programming languages themselves, even if it meant losing some detail on things like generating the chart of parser states. I hope they will consider a follow-up class that covers these subjects in more detail.
If you're thinking about taking the class, you may want to consider whether this class is important to the work you hope or plan to do. I've rated the class based on the quality of the instruction, which was very high, but in my experience, this material has not been as directly applicable to my work as, say, Udacity's Web Development course. You may also want to consider whether you're prepared for it. The only listed prerequisite is CS101, but I think you would find it easier going if you already had experience writing programs with trees or other large data structures. That said, the course was entirely self-contained and everything you needed was explained.
Here's hoping Professor Weimer decides to teach some more online classes!