At first it feels like a regular cold. But then you notice other things, and it's definitely worse. Don't get me wrong, it's still just a cold, but a bad one.
I had it while going through withdrawals, so I can't be too sure how bad it was, or if some of that wasn't my body raging against the lack of substance. It was for sure very unpleasant, but I'd do it again before I let some chode give me a shot.