Originally I started out with renshuu, but I can’t recommend it. The grammar explanations are very superficial. You go through levels without learning things in depth. The practice is repetitive: you see the same sentences over again. However, to study grammar you have to get creative with constructing sentences, and I felt that renshuu doesn’t provide an opportunity for such creativity.
Later, I bought a book that actually teaches grammar in a structured and methodical way (Panorama, I’ve mentioned it here) and I feel like I’m relearning everything from scratch, this time for real.