My Take
IXL allows students to practice appropriate math skills in any variety of areas. It offers a lot of great features to make it very feasible for students at a variety of levels, including changing the grade levels to letters (so students are working on Level B instead of Kindergarten--great if they are not working on grade level), having the computer read the question aloud, and data tracking for teachers.
How I Use It
I encourage students to use IXL at home for additional practice and over extended school breaks. It is also a great independent work task for students in my special education classroom.