Pros:
- Excellent review for intern year.
- Get it out of the way while you have time to study for it
- Step II is fresh in your mind
Cons:
- You have to study for it. If you take it later, you could probably wing it and do just fine.
- You waste perfectly good vacation days on a meaningless exam.
- You have no clinical experience, making the exam more difficult -> lower score in theory
In the end, I would say if you're going into something clinical, you might as well wait as long as you can to take it, so you don't have to study as much and will likely score better. If you're going into something nonclinical, I would say take it as soon after Step II as you possibly can.