I use Memrise in my classroom, but would like to throw some ideas around.
It is very beneficial to students learning, as it is engaging and competitive. However, from a teacher standpoint, it can be a little difficult.
Right now, I use groups for all of my classes and create courses for each week, where students get 20 words. The time feature is good, but I would love to see how many words students skipped, lock courses, and have a more teacher-friendly dashboard (such as seeing students who completed the course late).
There are other websites out there that do not stack up to Memrise. I know that it is not necessarily for classroom use, but making some of these changes would convince me to get a classroom subscription.