My daughter is currently in a Montessori preschool that we both love! That said, I agree with an earlier poster who said some are good and some are not. Most Montessori schools are private (there are a few public ones out there, but they're very rare) and so can and do vary greatly between them. I'd suggest you learn all you can about the philosophy and method, then tour some schools you're considering. See how it's put into practice, how the teachers lead and interact with the kids, etc. This is really going to be a gut decision for you.
Also, it can depend on the child. Montessori allows for lots of self-determination in work, and there are some kids who just can't handle the lack of structure. Many, however, respond very well to the opportunity to do one thing for long stretches of time, rather than stopping math "because it's time for reading", etc. So really be sure to consider how your child might handle it. And this will vary from school to school as well.
As for transition, my aunt has taught in a Catholic elementary school for 29 years, currently 1st grade but has done 2nd and 3rd as well. She has never had a problem with kids transitioning from Montessori. Just as each year brings new classes, new kids, new teachers, and new schedules, these kids had no harder time getting used to it than the others. The one thing that was different was that they had to get used to switching subjects when it was time to....but that was easy enough for most kids to handle. She's always had a fair amount of respect for the Montessori program and is contemplating adding that certification to her credentials.
HTH somewhat
Katie