We used it when it was still in beta trial mode and FREE (it was called education portal then). I refused to shell out $$$ now because there's plenty of other FREE resources available that my ds9 can use instead. When it was free, we did try to experiment with it over a range of subjects (math, history, science - not English though). It had a more traditional, structured approach to subjects, but I did really like how they broke down the geometry. Wish I had that type of instruction for geometry when I was in high school!
I've been un/homeschooling him for three years now and, to be honest, we've been trying to keep educational costs to a minimum. For us now, it's not worth it since we wouldn't be able to use it for all subjects. There's no French language, for instance, and the English doesn't have what my son needs. I can't remember if there was a lab with biology or not either - or with the other science courses.
At this point, I'd suggest seeing what you could get for free (or a nominal fee) or what study.com can do that other free sites cannot do. But I think this depends what you're comfort level is, what you're looking for, etc.
Also, there have been discussions about study.com on well trained mind forums (
http://forums.welltrainedmind.com/) if you want to see what others have said. After using the site for free, most no longer think it's worth it to pay for it.