Karate-Pops, burnt popcorn. You guys are lucky. I get a really strong smell that is much like a big waft of petrol (gasoline to Americans). Kind of a feeling more than a simple smell. I then hear something that sounds like an extremely high pitched noise, and feel very ill. My hands go clammy and I begin to spit, then I black out. Interestingly I also know about half-an-hour to an hour before hand. I just get a "funny sensation", like I have just had a moment of unreality.
On the issue of whether dogs can tell if you will have a seizure, I saw a documentary recently on TV that showed dogs can pick up tiny variations in a smell given off by some people with epilepsy. Dogs have been trained to alert others that their owner is about to have a seizure.