To add on: IXL generally requires around 28 right answers at a minimum to finish a task, but that isn't so many as to necessarily bore my son to tears, at least for problem types that can be done quickly, which is pretty much all of them at the lower levels. I believe it's been expanded to include coverage up to grade 8 now.
I wouldn't want to use IXL much for differentiation. I see it mostly as a practice site, which lets one jump around at will and which does its best to keep things from getting too boring. Unless you have a child who is good at teaching himself/herself on the fly, it wouldn't be good for differentiation in terms of advancing understanding or learning new concepts. Nor do the majority of tasks involve the types of in-depth problem solving that gifted kids (and I suppose all kids) need to practice in order to grow in math ability. I like IXL, but it really is geared towards skill practice.
