Hello, dreamer!
I think there are a couple of people who are either from Boston or live close to the city, but I'll just chip in my two yen from my several visits to the city.
It's a historic town, as you may already know, but maybe not very impressive since you appear to live in France. Knowing mainly Japan and the US West Coast, the city (and New England) has a totally different ambience from the rest of the US, I think. It is relatively safer than many major US cities. (I love cities where I can walk the sidewalks without constant fear of being mugged.)
Coming to Boston in September, you'll be arriving in a beautiful season with colorful autumn foliage, with lots of tourists flocking to New England.
The food is great--New England style, Italian, Chinese and lots more. I love dining out in Boston. (Japanese sushi is also great in Boston. Try the tuna there, even some of the best restaurants here in Tokyo serve sushi with tuna caught off the coast of New England.)
The drawback is the winter, when subzero temperature appears to be the norm. Lots of snow!! Moist hair and towel got frozen immediately outdoors!
That's my personal opinion. I heard from my kids that Swedes think the city is much warmer than Stockholm!
Anyway, enjoy your stay!