Fix-a-flat and other tire sealants just make a mess and really don't fix the problem. Its kinda funny, the person with the flat is happy to use the stuff, while the tire tech that takes the tire off is cursing that person for using it (I did it in my day). As for leaving that stuff in your tire, my understanding is that it breaks down the inner layer of rubber and makes it soft. If you ever feel the inside when a tire is removed with that stuff in it, it feel soft and pliable.

It is highly recommended that you get that stuff out of your tires as soon as possible tho. I know that it will cause vibrations being that the stuff will make a wheel unbalance able. When I was a tire tech, I would laugh at people that would come in to get wheels balanced with fix-a-flat in a tire. I would have to show the customer that every time I spent the wheel on the balance, it would come a different number because of the liquid inside moving around after every spin.

After saying all that, the short version...

The stuff will get you home, but pretty much make your best effort to get out of your tire as soon as possible.