With math, it seems to me that there's a need for learning material and a need for assessment/practice that's not too burdensome. It might be possible to get the practice from a website that's not so good for learning the material conceptually.
We currently use IXL for our five-year-old son. It's good in several ways: the material is keyed to state standards, it's easy to pick and choose which task you want to do (it's not in lock-step with a linear curriculum), the type of problems presented are varied when possible, it's adaptive, and going through a task doesn't take time if you know the material.
However, IXL is not be the best tool for teaching math, as it has no lessons. I typically have just told our son about a concept, then turned him loose on a task. Most of them he can finish in a few minutes each, due to the adaptive nature of the testing. So far we haven't encountered anything that I can't teach him easily or that he doesn't just "get", but that day will come.
Another area where IXL tends to be weak is on the explanations, if your child does miss a question. If that happens I just have him skip the explanations.
All in all, IXL is okay for practice. I would look at something else, like K12, Singapore Math, etc. for instruction. The Gizmos Explore Learning site looks cool, although it says it's a supplement only.
