I've never used them, but I've used a site called Time4Learning. It's got a wide variety of content (reading, math, science, social studies, etc.). I believe they offer a 2 week trial and it's about $20/month.
Back to Kaplan, I looked at the website and was a bit disappointed that I actually have to start a trial in order to get a feel for what the program is like. I'm also interested in the reward zone as a way to motivate my kids, but again, I can't really see what's involved.
I'd be interested in seeing more...I'd just rather not have to use my credit card first. My son finishes with his math tutor on Saturday and I'd like to come up with something to replace it with.