My Take
I like this site and recommend it as an alternative to some of the video game choices that our students move toward at home. The games are engaging and provide some learning aspect. Be cautious of free choice. Many of the students may get stuck on only going to one or two games and not exploring the variety of opportunities available to them. Getting "stuck" limits their growth opportunities. Use this as an occasional tool and not an every day.
How I Use It
Sometimes our students just need to play. This does not mean that it is not time for learning. I use engineering games as a "may do" activity in my classroom when their work is completed and/or they have earned some "free" time in class. The games are very engaging and the students are practicing a variety of problem solving skills in a very productive way. I find that it is best to direct them to a game at a time as a class before allowing free choice.