NO is fun, but not in a "cultural" kind of way really. If you're looking for some good drinking fun and some spicy seafood, it can be great. I've had fun on the weekends I've been. The major party holidays though are incredibly packed, especially Bourbon St. The downtown isn't bad, but you can quickly end up in some ghetto areas.
Austin is probably the best for live music and some more intimate pub scenes, but a lot of it is blues and classic rock. I'd see who was playing at Stubb's, just to experience it, as a lot of big names come through there. 6th street is the easier "I have no idea what to do or what to see" area, and it isn't terrible. If you're into strip clubs, the Yellow Rose is probably the best.
I've never been to Portland, but I hear it is a lot like Austin. Also, hear that it has a high concentration of hipsters; which, I imagine Austin suffers from as well, although I haven't been there in some years.