I have joined brilliant tutorials 10th Target IIT and Elite for IIT-JEE 2006.
1) I found that the notes of Brilliant Tutorials are much limited in Scope.
They do not extend the scope of syllabus. Before reading the notes of Brilliant you have to refer your textbooks or otherbooks. You will hardly find some extra comments or topics explained in detail. This is the particular drawback of this tutorial sets.
2) The other major thing which should be considered is that time at which topics which are taught in school/colleges do not match with Brilliant. The topics which are covered in 12th in my board(Maharashtra borad) appear in the first few sets of Brilliant. Thus...you can understand...
3) The biggest advantage of joining Brilliant are the problems. The problems are very good and the solutions are readily available and the solutions are very well written.
4) Diagrams, figures are not so proper and not frequently used.
In short, brilliant tutorials has very good problems and their solutions but the concepts cannot be gained 100% from Brilliant. You will have to definitely refer other books.