Membean is a wonderful way to learn. I'd definitely recommend it, however, there are a couple things I'd like to address.
My overall take on Membean as a student is mostly positive. It's incredibly easy to use and is good at what it was made to do. But again, as I said earlier, Membean is mildly sexist in its definitions, so if you have a strict ideology about sexism, I would be hesitant about allowing your students to use Membean without using it yourself.
How I Use It
First thing first: I'm actually a student who uses Membean to learn new vocabulary for my English Honors class. I really do enjoy using it, and it does its job well. It's incredibly easy to learn from it. However, I have noticed that it uses gendered pronouns where there should be none. A professional definition would use "they" as the only pronoun for non-gendered words, but Membean uses he/him and she/her instead. Normally, I would have no problem with this, but the cases in which these pronouns are used are not random. The pronouns used in definitions for "good" words are "she" and "her," and the pronouns used in definitions for "bad" words are "he" and "him." While only mild, this is still sexism and is unprofessional.