Fixaflat stuff is worthless, IMHO. That stuff never worked for me.
Spare tires are like seatbelts -superfluous until you need them, and then, boy do you need them.
Personally, I don't like to drive without one. A year and a half ago I had a gf who blew out a tire on her Z-3, and those cars don't have a spare and instead rely on "run-flat" tires. Point is, it took us 6 hours to get the car towed to a shop, which didn't have the tire, and we had to wait 2 days for one to be airborne expressed to the shop. We could have at least been on the road after 15 minutes to get to where we needed if we had the spare. Instead, we were at least 6 hours away from a rental car.