If you already have a foundation then Italki is great... I studied Chinese at college (high school), which helped me with the pronunciation and basic grammar. After that, I started using Italki and it's been by far my best resource. So although it might be a bit overwhelming for beginners, it's very useful for those with at least a foundation.
I know your question is specifically about Japanese, but I feel having studied Chinese (a tough, completely different, north Asian language) our experiences on Italki will be fairly similar! :)
I should add that I've mostly used Italki to ask the community questions, start discussions and book lessons with community teachers. I haven't really explored the 'language partner' side.
So to sum up, it's a fantastic resource for those coming off introductory courses to really grow and expand their skills. As for reaching complete fluency, I honestly feel like that's best achieved by spending time in the country (although there are exceptions). :)