If I go again--I would stay nearby and plan two days. The commute back to Paris for us about killed us we were so exhausted. We should have used our planning practices as we did back home and felt really like a tourist who had never been to Disney before.
We went in Spring (I think it was actually on hubby's birthday even in mid to late march). The day we went actually was more crowded than we expected.
Also--just a generalization, not meant to offend...it seemed queing up was a problem. We'd get to an attraction and there would be this huge crowd in front of it not moving. I just didn't get that. Once you burrowed through--you'd see there was little or no line for the attraction.
The CM's will speak English--I actually felt kind of bad for them doing so. I wanted to speak French (what little I knew) and they would not reciprocate. In fact, one even said "You're American aren't you".
We did have fun in the park--but we had some moments that proper planning and research would have avoided. The train ride in was easy. We had stroller age children, so we really should have brought the strollers. We rented them at the parc and they were a big PITA. Also, carrying back sleeping children on a one hour train ride and then subsequent metros wasn't much fun.
Splurge and stay on site!
(oh and despite my negatives--totally go for it. We were in GB and France for a total of two weeks. The kids were totally excited when we got to Disney. I think they for a minute thought we were back in Orlando).