We use it for my older two kids - currently 2nd and 5th, but we've now used every level, k-5.
I thought that a good 50% of K math is what I would consider preschool level - before and after, which comes first, etc. However, after that it seems to move pretty quickly. What my dd is doing now in 2nd grade math seems more advanced than what the second graders were doing at the end of the year when my son was in a brick and mortar school.
As far as how I like it, I think it's okay. We don't usually do full lessons - my kids pick up quickly, so we rarely use manipulatives or anything - I usually just explain it to them and we move on. The assessments seem to be a good guage of understanding. The place they break down is if the child doesn't pass the assessment the first time around - they don't have alternate questions to have them try again after reviwing. However, it's not usually too hard to make up alternate questions. I'm not sure if it will be as good a fit for my youngest son. We're not sure if we're going with k12 for him next year or not, though.
eilonwy, what have you seen of the new k12 math program? I've been curious about it, but I don't know if they have any samples up online anywhere, yet.