I'm not sure how helpful I'll be since I'm 2 days into pre-k, but so far, we love it. I made the decision to go with Calvert about 2 1/2 years ago, and I've just been waiting until my kids were old enough. I bought it for my son, who just turned 4 in August, however, my daughter (3 this month) has been sitting in and doing all of the activities with us. I've found that it's easier having 2 doing it at once, since they play off each other. There are also a lot opportunities to act out parts of stories, and it's helpful to have more than one child to do that. For example, we read "The Little Red Hen" yesterday, and my daughter was acting out the part of the hen, while my son, a plastic pig, and I were sitting on the couch acting out the parts of the other animals. Tomorrow they are acting out "Jack and Jill" so we're going across the street to our neighbor's hill.
It takes me about an hour to do school with them, but I add in Hooked on Phonics. I'm doing Discoveries in Music with them as well, but we do that on Wednesdays (we don't do regular school on Wednesdays because we have lots of appointments on Wednesdays).
I like the way it is set up. You do an opening, then there is a discussion. There is usually a story, some sort of art or craft, sometimes a math activity, and there is time to play at the end (that's usually when the kids need to act something out or play some type of game). It's cute.
I also had a difficult time deciding between K and pre-K. Intellectually my son is ready for K, but he lacks the fine motor skills and attention span for it. I added the Hooked on Phonics so he would be challenged. The thing is, I've found that he IS challenged with pre-K --not so much with the material, but with the arts and crafts. Due to his fine-motor limitations, he usually tries to avoid them. He's doing them with Calvert, enjoying them, and making progress. I think I made the right decision enrolling him in pre-K this year. Every child is different though, so you need to consider your child's strengths and weaknesses before making your decision.