Great question! We've mentioned Comcast's Internet control and bandwidth cap restrictions before, and we will tell you that they are quite serious about the 250GB cap. If you violate it more than once in a year, they will terminate your account without notice.
That said, here's the breakdown of the bandwidth you speak of:
Netflix Movies (HD): These guys are around 3.8Mbit, which means it's about 3600MB for a 2 hour HD movie.
Netflix Movies (SD): Each of these movies are around 500-700MB each, depending on the length of the movie.
Netflix TV Shows (HD): A 30-minute TV show will be about 1500MB.
Netflix TV Shows (SD): A 30-minute TV show will be about 400MB.
If you do the math, that means you can stream about 3 seasons of Entourage and 40 movies in HD. However, that's not to account for standard Internet usage, so with YouTube, Facebook, and all of the other things you and your household may use, that's looking to drop the number down to about 20 HD movies, which is still a substantial amount for most people.
On the other hand, if you're sharing the Internet within a small apartment complex, you'll be hitting these bandwidth limits much faster and easier. It's probably better to invest in a more expensive Comcast package if that is the case, as the limits raised for small business operation.