I've had Mandarin lessons with some independent teachers and a few institutions while in China, but my experience at That's Mandarin stands out as being particularly excellent.
My teachers were very patient and thorough. They were constantly attentive to my needs and progress, and made sure I was comfortable asking for clarification, asking them to repeat something, etc. I was also free to ask about miscellaneous things that interested me that didn't necessarily stick with the lesson plan -- I would be given a satisfying explanation and could walk away with some extra vocabulary or a clearer understanding of some grammar issue that was bothering me. For students who engage better with the material when they can participate more actively in the lesson, this style of teaching should be perfect.
That's Mandarin also provides access to their online resource NihaoCafe, which seems well-designed and well-built. Teachers use it to take notes on what was discussed during class, and students can access those same notes again at any time. I found it an excellent way to reinforce the material I learned during class. There are also some pre-built quizzes you can take, which are also helpful for retaining the lesson material.