As a brief summary of the company functionality: teachers upload lesson plans onto a central server and other people can go online and pay to download them and use them. Essentially, it monetizes the idea of lesson plan creation and distribution.
There are similar repositories that exist across the Internet without a pay-per-use structure: Teach For America used to have one that a friend of mine gave me access to briefly; The American Federation of Teachers has its own called Share My Lesson; and there are a variety of smaller companies and non-profits that offer teachers free options of lessons to use.
My main gripe of this kind of company is similar to the thoughts of one upset blogger who wrote, "I embrace the sharing, the collaboration and the freely giving of resources that the math teachers do on Twitter, their blogs and the internet in general." When someone adds a price tag to something that could foster collaboration, it begins to erode the system that's supposed to be a public good.
In my opinion, my lessons are and should be shared widely for use; and the same for lessons of other teachers across the country and the world. I have advocated for a platform like that in the School District of Philadelphia as I would in New York City as well. I know that some people have added to their income by selling items on websites like these but in the long run I think it will benefit a few adults and children will pay the cost.