I think that's a great move by Udacity. I am trying to catch up on unit 6 right now in CS101. It's definitely a challenge to keep up if you are doing the course as a "hobby". Of course I also don't really care about the certs so as long as the course material is available I'm happy.

For noobs like me I highly recommend the CS101 course, I found that a lot of stuff I was struggling with in Learn Python the Hardway (a great resource as well by the way and many thanks to Zed for making it available) and other sources online became clearer to me during the CS101 course.

I am looking forward to CS253:Web Application Engineering next.

Another thing to keep in mind is to avoid "over registering" by trying to do too many courses - some courses are rather intensive/ time consuming.

Who grades the homework assignments?

They're graded by a computer program. Usually you are writing function(s) and need to return a correct value.

As CS373 progressed it got more complex: you worked from some helper code (like a matrix class) and had multiple test cases to test your program.