I've used this site as a tutorial in my basic writing courses. Students find it easy to use and helpful once they get the hang of it. It's also a great link for students to work from at home. Some students struggled a bit at first understanding how to move throughout the site, but with a bit of help, they were soon pros.
Students found the design fun and were eager to try it out, even if it was "grammar." They liked to compete with one another on certain assigned modules, or modules they explored on their own.
With my transfer-level writing courses, I've used this site for students who need some brushing up on grammar and usage. Again, it was ideal with this because they could do it on their own at home or in the writing center. If they got stuck, they asked me and we could work through it together.