Several friends work at/worked at MS. Like most I-banks, the culture varies from group to group. I have heard good things about a group over on Sand Hill Road in CA and I have also heard that the some groups in NYC are a bit more intense/uptight.
Remember that MS was the original 'white shoe' firm. This carries over into how they behave internally and how they carry themselves. MS does not have a rep for being super-humble. They see Goldman as their main competition. But YMMV.
Haven't worked there myself, so take my opinions with a big grain of salt.