In a review of Vocabulary.com, University of Michigan English Language Institute lecturer Pamela Bogart called the site her "favorite vocabulary & dictionary site/game." The review appeared on her blog, Connected English, where she writes about free English learning websites, apps, and "tips on how to leverage our connected world to improve your English communication skills every day."
About Vocabulary.com, she writes:
You might call it a dictionary with witty definitions. You might call it an advanced vocabulary game that can quiz you on up to thousands of words by definition, example usage, and spelling, and keep track of which ones you’ve mastered. You might call it a massive database of real examples of how words and phrases are actually used. You might call it a way to learn all the forms of a word and their relative frequencies of use in English. You might call it a resource full of useful wordlists that you can bookmark & learn. You might call it a great place to make your own vocabulary lists and quiz yourself on them.
See the full text of her review here.