Because I was unsure about the amount of fizziness the ginger ale would have, I put the bottle in the sink before uncorking it. With one hand over the top of the bottle to serve as a splash guard, I lifted the lever. A powerful jet of frothy ginger ale whooshed from the bottle, rebounding off my hand and splashing forcefully into the sink. The hissing fountain lasted for several seconds. The atomized beverage, strong with the essence of ginger, stung my eyes and my lungs. When the eruption subsided there was just a few ounces of liquid remaining in the bottle. I poured it into a glass and took a sip. There was no sweetness, and curiously, not much of a ginger taste either. I asked my wife to sip it and she almost gagged. She said it had an intolerable sulfur smell and taste. I hadn't noticed it but when I tasted it again I could sort of convince myself that there was a sulfurous tang.
I figured I'd used too much yeast, so for the next batch I used just an 1/8 of a teaspoon for one half gallon of ginger ale. I also used plastic screw top bottles instead of the lemonade bottle. I used a cup of sugar and about one-and-a-half times as much ginger as before.
UPDATE: Mark C posted what looks like a really good and safer ginger ale recipe.