Yeah I think CS50 is too basic. The C part is interesting though, has some minor projects that worth working on. For anyone who has some programming experience (whatever language), I think CS61A is a better course.

I really wish there is a course that uses C (like first 4-5 classes in CS50) and uses C ONLY and then go as deep as possible, similar to CS61A, but apparently on other topics (one system programming project?)

The problem sets in CS50, at least the first half, are great and I would highly recommend. I ditched it when it moved to python and web, the problems were interesting but there was an added level of boredom in the 'now make it a website' approach.