plenty of features, lots of sentences in a language pair (many "taken" from tatoeba, so they say under "source")
but as soon as you "touch" anything like an additional function, such as adding your sentence to a "collection", or reading grammar a second time (first time works fine) .. almost everything you do, and you get prompted for the PRO version.
in the free version there is also the daily limit of 30 sentences (which you hit in no time)