My Take
I believe IXL is a great platform for homework assignments, or even quick in-class assignments after a lesson. It is a great way to reinforce what students learned, as the questions are usually quick, and get progressively harder as your score increases. I do not believe it should be used as a way to learn, but more as a way to confirm what level you are at, and how well you understand a topic.
How I Use It
IXL has been a part of my schooling throughout elementary, middle, and some of high school. It offers a very wide variety of topics and questions, especially for math. I've only used it for math, and it does the job pretty well. I've always used it after being taught, not as a way to learn.