Not experience, but I've looked into also and have a couple of observations.

Using a small gasoline engine makes practical sense since it's cheaper, and a gas tank carries a lot more energy than a battery. However, the cheap 40-50 cc engines have a reputation of wearing out quickly so I think it's a false economy. The noise and heat are drawbacks. Engineering it is a little tricky because you have to mount it somehow down around your feet, and it seems to me that there's not a lot of room there if you want to retain comfortable pedaling, with pedal assist being pretty much out of the question, and finally I have to wonder if most bike frames are going to handle the vibrations long term.

In short, it appears to be a short term novelty modification. In fact, if you read message boards where people have done this, the guys bragging about the durability have gone 400-600 miles. I don't know about those folks, but 500 miles is a month's worth of riding for a cyclist, so that's not really a good application. So, personally, I've never really been tempted by the idea.