rigorous and well-designed course

I used TTP almost entirely for quant and couldn't be more satisfied. I had major math content gaps when I started GMAT prep, but I learned SO much with TTP. Their quant course/lessons gave me the tools I needed to deeply understand the concepts, and then use that understanding to answer GMAT questions correctly.
What really set TTP apart, in my opinion, was the amount of high-quality, GMAT-style questions included throughout: there are questions in each lesson as you're learning and at in the end of each chapter via chapter tests. Then there are even more strategically woven across the curriculum (e.g. review quizzes on previous chapter content before starting new chapter, and cumulative tests strategically placed after every 3-5 chapters).