I've been using Edmodo for four weeks now, and am liking the way it is streamlining homework submission. It takes a little getting used to. Most kids will need some training in uploading their first submission. The most common mistake is that they send their homeworks in messages rather than as submissions. The way the the grading is done online is neat and it's easy to export in CSV format. You can add comments to each marked piece of work but I can't see a way to review these comments once you've assigned the grades. I've previously tried Yacapaca, but the administration was more complex. With Edmodo the kids just sign up to each class, so there is no need to generate of class lists, usernames and passwords.
I came across a similar product called Soshiku which I has all the same functionality, and with a slightly cleaner interface. It also has the advantage of offering collaborative assignments. Unfortunately, unless you are happy to limit yourself to 20 assignments you will need to pay USD 5 per month.