I have studied with Assimil before. Many years ago, French, all by myself. After a few months, I spoke to a French person that could not believe I had never been to France.
So, after checking several methods from the public library to learn German, remembering my good experience with Assimil, I bought the German version of it. Well, I am definitely not disappointed this time either! What I like about Assimil, is that you really learn to speak the language little by little. I love the way they introduce the conversations, especially for an English speaking person. They sort of ease you into the language like no other method out there. The trick is to repeat each lesson until you learn it completely. I write it over and over, also, to memorize it. I also love the fact that they don't overwhelm you with the grammar from the start, which I find very discouraging.. The book is small and light, so I take it everywhere.
I definitely recommend it. But I strongly suggest to learn each lesson by heart, like an actor learning his/her lines. That really works for me. I've been to France many times and now speak fluent French, and I plan to do the same with German.