It depends on what language you're learning since the web is english-based. It is useful if you're learning English, or if you're an English speaker learning another language.
If you're a non-english speaker learning a language other than English, then you must first achieve proficiency in English. Or find another method to study.
I see it as a digitalised and upgraded version of traditional workbook practice. It's great for drilling vocabulary, sentence structures, genders, conjugations etc. into you. I use it for German, Dutch and Italian practice; but just like workbooks, I think it's only supplementary.
i have been using it for 10 months to learn Italian. I think it is pretty good. I try to listen to music and find videos of people speaking in italian to help my understanding of the speech and i read online newspapers to help my reading, but I haven't really advanced in speaking the language (which is why I am here).
I've taken a look at it just now. Marcelle put the finger to the heart of the matter.
if you want to learn vocabulary and grammar, its a good method to learn any languague