Have been to multiple Saturday birthday parties at this Gymboree - including my daughter's 3rd birthday party. Typically 90 minute parties, I believe for a reasonable price per kid. You provide cupcakes, ice cream, juice boxes, etc. I believe they can even do theme parties if you want. It's a great place for kids to expend some energy.
The children receive a good mix of free time to explore, climb, run, etc balanced with organized activities led by one of their employees. Our daughter loves it. I would imagine some of the success depends on who is leading the activities - there is no one size fits all, meaning some instructors are better with certain kids and certain aged kids.
The organized parties feel a little rushed given all of the activities and free-play time they want to cram into 90 minutes and I wish there was more than one instructor/staff member onsite at a time, but other than that, I strongly recommend for an inexpensive alternative to backyard birthday parties.