One of my best friends gets them regularly and he describes it as feeling short of breath, like a weight is on his chest, gets dizziness, heart pounds and all he can think about is that he's having a panic attack.
I've only had one recently, a few weeks ago when I was v stressed, and I can agree with the weight on chest, shortness of breath, dizziness and heart pounding. I also felt like the veins in my neck were gonna burst and it's like there was a mental block in my head, couldn't think about anything. Like an extended state of shock.