I chose to do IES because my school's program in France does not include a homestay or the opportunity to take classes at a French institution. My school told me I might not even get credit for the classes I took with IES but I chose to do it anyway, and I am very glad I did (I did credit actually).
If you choose (you can also live in an apartment with other Americans) you can be placed in an apartment in Paris with a family, which is the main reason I wanted to do IES. I had a lovely family in a really great location and the mom was a fantastic cook. I probably wouldn't want to stay with a family for more than a semester though- by the end I wanted to feel more independent in my home and I felt a little bit like a stranger still, but I did really love the home stay. I didn't totally click with the other IES students, but I made French friends through my host mom's son, which was really fantastic because it really helped my French (and they were great people).
I absolutely loved the IES classes I took- 18th and 19th century art history (with trips to the Louvre and the Musee d'Orsay) 20th century French literature, and translation (that was a bit difficult). I took two classes at the Sorbonne- I almost wish I had only taken one because the classes were very difficult to follow and had a lot of assignments. I did perfectly fine in them though so it's definitely doable. I also learned a lot in those classes and made a couple of French friends- also great.
I had a lot of time to wander around Paris by myself- you can really create your own experience with IES and feel independent, which I find very important. A lot of people traveled during the weekends - I visited a cousin in Florence one weekend but other than that I stayed in Paris.
The IES staff were always extremely supportive and nice and I really enjoyed all the programs they had in Paris - a few hangouts with French people, trips to the Opera and Ballet, and a trip to a chocolate competition thing - that was amazing. The superb quality of the staff was really one of the best parts about my experience.