We've really liked their approach as a complement to regular school math. Their classes really show how mathematicians think and solve problems in a deeper way than kids are typically exposed to in school.
The teacher typically starts the class with something engaging and accessible, like a physical puzzle or simple game, then uses it to illuminate abstract mathematical concepts like parity or the pigeonhole principle. In addition to asking questions for the kids to figure out, the teacher encourages the kids to formulate interesting mathematical/logical questions about the topic themselves and work with each other to explore them both intuitively and logically. I particularly like that the teachers are good about giving them space to try different paths to solving a problem before giving them guidance towards the solution if they don't figure it out on their own.
Overall, I think they do a great job of showing what real mathematical problem-solving is like, and how fun it can be, beyond the mechanical aspect that tends to get emphasized in schoolwork.