Actually, I'm an NU freshman who just recieved my bid last night (kappa alpha theta!). I won't go into the so-called "reputations" of the different sororities, because while there is always truth to some stereotypes, everyone has different experiences and is looking to get different things out of the greek life.
With that said, the Greek system (for girls, anyway) is different from that of other schools. While it is a large precentage (about 38% I think...), all the girls in the different houses remain/become friends with each other--it is very unexclusionary in that sense. I have friends who have gone greek at other schools and they wouldn't even consider talking to a girl in another sorority.
Rush, however, remains very traditional and is definitely an intense experience, but I can already tell it's going to be worth it. It works out most of the time (there are exceptions--one of my best friends ended up dropping rush after being cut all of her choices) and people tend to end up where they need to be. There's a sorority for everyone and the range of houses is really diverse. I also like how many girls proudly wear their letters around campus.
In terms of the negative, if you end up in a dorm such as Bobb or Elder, most of the girls are going to be rushing and if you choose not to, or you end up dropping, then you might feel really left out for the first couple months after rush. But statistically, there are more girls not in sororities than those that are. It's totally up to you what you want to become involved in.
Feel free to PM with more questions.