I have had the pleasure of piloting IXL in all my classes. I really enjoy it because the topics offered in each class match up quite well with what I teach. It gives me great reports so I can see how well each individual student is doing on certain topics. It tells me if the student is performing below level, at level or even if they fully understand the material. In fact, IXL provides 35 different reports. I can go so far as to see which problems they've missed. Furthermore, if a student misses a problem IXL shows them how to do it.
There are two other things I like. Since my pre-algebra class has problems working with fractions, I can move them down to the 5th level to learn to work with fractions. I started them on the adding and subtracting fractions with unlike denominators using manipulatives so they can see precisely how the lowest common denominator works. In addition, I do have one pre-algebra student who is at a level between the pre-algebra and the algebra class and I've got her doing some 8th grade work.
The second thing I like is that students earn a reward after 5 min of working with IXL They see pieces being turned over on a large board and it helps them see how they are doing.
Overall, I am really liking the data I get from IXL because it is helping me create better instruction that meets the students needs.