I'll be starting an MBA while in fellowship.
It makes sense for some folks, not for others.
There are a lot of good conversations on this buried in the MD/MBA subforum.
In my opinion, for the great majority, obtaining the MBA during medical school makes less sense than completing it later (i.e. closer to when you will be actually using those skills/network). 4-5+ years later, those things get stale.
Also look at your goals and juxtaposition it against the cost. Determine your odds of achieving your potential ROI. If you like those odds, take the plunge.