My 20 month old dd loves gymboree, but I don't think it's worth the money.
I don't know your story but it can be an expensive faff getting out of the house to make it to the class, if your child is ill more than once during the 12 weeks you start to miss classes that you've paid for. You can make up classes on other days at the one I go to but that eats into the rest of time I have with dd.
They do explore different themes, but it can get very repetitive (fun for the kids, not for the parents!). The membership fee does include open gym sessions when you can just turn up, but I find at the one I go to they are at really inconvenient times.
I also don't find the other parents that friendly but that may just be the snobbishness of where I go (near notting hill, so lots of nannies/people not interested in making friends).
They should offer a free taster session before you commit to booking a whole 12wk period, so give it a try and see what you think. £150 is a lot for 45 mins a week, but if it will take the pressure off you for a bit then it may be worth it. My local one also does 8 week packages.