We first heard of this through Homeschool Buyer's Co-op. We watched all the free videos we could and decided it would be worth the smallish monthly fee to get a subscription. We love it.
But what is it? Brain Pop and Brain Pop Jr. have many, many, many short animated videos on various subjects. We've watched videos on the brain, snot, aliens, the water cycle, motion, classifying animals, and arrays. Actually, we've watched a whole lot more than that, but I'm pretty sure you don't want me to show you the entire list.
Brain Pop is geared for kids in 3rd or 4th grade on up. Brain Pop Jr. is for the younger set. My kids (ages 9, 9 and 5) have watched videos from both sites and have been able to understand them. There are short quizzes your child can take after each video as well as games and activities.
I usually turn it on during breakfast or lunch and they take turns choosing videos (they are 3-5 minutes long). It's a fun, easy way to get some extra learning in or to review a subject we've already learned about.
Thus far I haven't had any problems with content aside from a little "save the world and go green" stuff. I get a little defensive about how hard they push that on kids. Other than that, I've been okay with everything.
There are multiple free movies you can watch on either site and they change every week. Check them out and see if this is something your kids would like.