It was a great experience. It wasn't just me, though. I went to the show in St. Louis with my friend Nick, and during the last song of the set, Slug came out into the (admittedly smallish) crowd and shook hands with people, etc.
After the set ended, he signd autographs for people and chatted with me, Nick, and a group of about 4-5 other people. Slug took us back behind (kinda underneath) the stage and handed us some "Free Beret" t-shirts. I don't even know who Beret is, since I'm just a casual fan who had only heard 2 Atmosphere songs before Warped. But right after that, Slug asked if anyone smoked bud, and when everyone in the group enthusiastically answered "yes", he whipped out a packed chocolate philly and sparked it up.
I really didn't say much to him since I'm somewhat shy (social anxiety disorder) and as I said, I didn't know much about the music at the time. Still, it was an awesome experience. After smoking up, I went and got tossed around brutally Flogging Molly's mosh pit.
That's god damned cool, man. Nicely done.