We're big Gymbo fans. I took DD to a free class when she was 4 months old and all she did was lie there and look at the Gymbo clown doll they had. I didn't think she got much out of it so I waited until she was 6 or 7 months old before I took her back. We've been going ever since then (she'll be 2 next month). We were in gym class but recently switched to one music class and one art class/week. I'm PG with #2 and didn't feel like chasing her around the gym area anymore.
Depending on the age group your DC would be in, they basically consist of initial circle time - sing a song, do a few mommy/caregiver and baby exercises, play with the toy of the week. Then they move things to the gym area - play on various equipment, focusing on one thing for 2-3 weeks at a time. Then it's bubble and parachute time, then a Gymbo song, then a good-bye song. I think that's it. The art and music classes are different. Let me know if you want to know about those.
For the life of me I can't remember the cost of the classes. You'd think I'd know by now. I usually enroll when it's Gymbucks time so that I can earn a couple to buy clothes with later. It is pricier than some of the other programs - Kindermusik, park district, etc., but we really like it. You can really get your money's worth if you take advantage of open gym. Our gymboree had it 6 days/week this summer, but have cut back to 4 days/week after Labor Day.
DD loves Gymboree. She remembers the songs, we sing them at home, she likes Gymbo (you'd think most kids would be afraid of a clown), and I think the various classes have really benefited her development.