My HG+ DD attended Montessori preschool for a year and it was a good experience for her, but I'd definitely say that any child's experience will differ greatly based on his or her personality and the school itself.
For instance, my DD was in a 3-5 year old class, which was a decent fit because she was not quite 4 at the beginning of the year. She was the most advanced child academically and the older kids were closer to her socially than age peers were, although my DD was still more socially advanced than most. So, the mixed age grouping was a benefit.
Also, DD was able to work at her own pace and progress through their series of activities in each of the different realms (language, numbers, etc.). The only thing was that she had to start at the most basic level of every activity and show her mastery before being allowed to move to the next, so it took her some time to actually be allowed to work at her level. I thought this would drive her crazy - and it definitely would drive a lot of kids crazy! - but she is the stereotypical girl who will hide her abilities and not act out. So, it's a very systematic approach to learning and might not work well for every child.
Still, having had my kids in several different preschool types over the years, I think the main difference between Montessori and all the other programs was in it's systematic academically-focused nature. The other programs all worked on art and very basic writing and circle time and play time, but it seemed the goals were more exposure to these things than mastery of skills.
And that is where your child's personality comes in. My DD loved to "do" school, so going through the motions that year was reward enough for her. Other kids may actually want to be challenged academically, while still others may want nothing academic out of preschool and would prefer something that was entirely play-based.
