Will Boehm (Chew, 8k to 3k)
I studied with topazg from 8k-3k (just over half a year) and it was one of the fastest periods of go learning that I have ever had. He gives highly personalized lessons that greatly helped me with some of my weaknesses. Most of all, I feel that I am much better at direction-of-play issues, and I lose less to opponents who rely on drastic overplay. Topazg spends massive amounts of time on each lesson, before, after, and during the paid period. While there are always stronger go players to learn from, topazg is great at conveying the ideas he teaches, so you learn and master more ideas, faster. Topazg's new videos seem to cover the same depth of information as his lessons, though they are shorter because there is less interaction. He has a relaxed and informal style, and I recommend his lessons highly to anyone within his recommended teaching rank.
Guillaume Gallais (Actorios, 7k to 4k)
I've taken 8 three-hours lessons from Graham from February to June 2010. I was 7k when I started those and I used to always play in the same way at that time: territorial and solid, building up all of my games from komoku, looking forward to building shimaris. Graham helped me get more flexible and versatile through showing me usual responses to pincer plays and bringing me to play in a lighter and more forcing way. It didn't totally change my gaming style but he helped me build confidence that I could deal with different situations if needed. I really did enjoy his attitude towards teaching: generating a dialog over decisions which brought you into such or such position: you could then understand if it was your idea of its application which was flawed at that time. I find this approach all the more pertinent that everybody has a personal style preference and the teacher and his pupil may not share the same. Even if getting a totally different point of view is fruitful, i don't think you can change your demeanor from one extreme to the other. You have to build from your strength as well as reducing your weaknesses. As a result of those courses, I managed to gain 3 stones in strength over time (Up to 4k on KGS which I reached a couple of weeks after my last lesson). Still, the courses won't get you further by themselves. You'll need to apply the principles in your games: testing what you've been taught - which also mean at the expense of some defeats - will allow you to rub them in little by little. It was uneasy for me at start, to understand that in order to progress, I had to enter an area of discomfort. Graham did help me cross the line. This was my first experience of formal online teaching. I'll clearly consider doing more of it in the future. I do recommend taking lessons from Graham if you consider that you need an external impulse to reach over a plateau and if you're 6k or below in strength. Graham is friendly and fostering. He is also caring about the added value of the courses, alway looking at giving you enough "bang for your buck", using his own words. He would let you know when he feels he cannot teach you more than he already did.