I went back and got my MBA from a local (at the time) business college. I did it for several reasons, most of which agree with yours -- plus my company paid for it. I wasn't looking to leave the company as much as make sure my profile stood out.
In the grand scheme of things, I don't know that it's helped my personal career that much, but I do really like having it on my resume. And what you do appreciate are the nuggets of knowledge you gain from taking the classes in the process. However if you are already managing projects, some of these topics will probably be repetitive compared to your existing experience...while the classes may offer a little more depth.
It took me about 2-3 years taking 2 classes at a time in night school, per semester. I'd still do it all over again, but might choose a higher profile school -- or one of the "Executive MBA" programs instead.
All that said, I have to say it was a lot easier to complete than I thought. Engineering school was 1000% more difficult for me.
In a nutshell, you won't regret it one bit either way.