My kids have gone to Gymboree Play and Music since they were newborns. I took my son when he first started crawling, although they offer classes starting at birth. My daughter when to playgym when she was a week old, although, it was more for my son to have some special playtime with his new baby sister. My son also was enrolled in Art Classes there and had his 3rd b'day party there. We invited lots of friends and the party was everything I could have dreamed of and more.
I really can only praise Gymboree - they're just wonderful! The only thing I can say negative would be that I feel that since they have raised their prices, they are incredibly OVERpriced. I realize they are now offering a wider variety of classes but with my children being a little older now (3 & 5) and not having an unlimited budget, we cut Gymboree from our regular activities when the prices went up.
They do give free class passes, one per year per child, to try a class out before enrolling your child. So, if you're considering Gymboree, go check it out for yourself - there's so much to do! And, the kids will LOVE it!