The variables obviously are Where are you getting the MBA (Ivy League MBAs, even evening ones, have more value), what is the price tag - there is a big difference between a $50K MBA and $200K MBA, and what field you'll be going into.

MBA has very little (some might almost say negative) value in Silicon Valley startup world, but obviously more value in mature companies, or companies that are transitioning from startup->mature (possible exception being the CEO, VP of BizDev/Marketing and possibly VP of Product Management). MBAs have close to zero value for people who wish to be simultaneously tactical and technical. I've never seen a network engineer, or firmware developer get any value out of their MBA. It's really for people who are either going to go into the business side of a company, and/or go into management.

Also - it's important to note if you'll have to quit working while you are getting your MBA - that makes the credential doubly expensive.