Being a great mathematician doesn't necessarily make a great math educator. It is odd Tao chose Durret, but I assume it's due to the book being freely available online.
As the blog comments suggest there's a difference between a good self-study book and a good textbook for a class. The book will be accompanied by (at least) what look to be a good set of course notes.
Some googling around suggests his students are quite pleased with him. He has, according to Wikipedia's intro for him, the undisputed king of math blogs. Both of these point to him being at least a good or above average educator.
