**Here's some issues I have with fix-a-flat:**

 - In the case of a large puncture / fast leak, you'll need to add air.

Runflats are good because in case of a sudden loss of pressure, they'll get you to a tire shop. With fix-a-flat, if you have a sudden loss of pressure, you'll have to limp your car to a gas station or carry a compressor around.

 - Fix-a-flat isn't great for your tires, either.

See [this][1] answer to a previous question - but essentially, after getting to a shop after using fix-a-flat, some work might need to be done in order to get the tire properly balanced, since the fix-a-flat "goo" can throw the tire out of balance until it is removed. This actually isn't a huge deal, it just means a little more work at the tire shop, and you need to remember to bring it up (why I'm mentioning it).

Finally, with fix-a-flat, you'll have to spend some time on the side of the road / at a gas station putting a band-aid in/on your tire. Runflats just... sort of *go*.

**Given those three things, here's some issues I have with runflats:** Runflats are only good for, what, 100 miles? And they also need to be replaced more frequently (treadlife is worse), on top of being the more expensive tire like you said. Also, when they do go flat, you generally need to replace them in pairs, in my experience. To top if off, (at least the older models of runflats) would blow out at low pressures, so if you had a slow leak, you're completely toast. Meanwhile, here I am, I've driven for over 3 hours with a slowly leaking tire where I started the trip at 32 psi and ended at 20 because it had to be done (normal tires).

**Answer:** Since you're in a situation without a spare (as is my mother, and this is what I tell her), **yes, I believe that fix-a-flat is good enough for you (depending on your situation, this is somewhat personal preference) in lieu of getting runflats.** Runflats are good for 100 miles. If you're on the road and really need to go and get going, fix-a-flat, while not perfect, will get you where you need to go, no matter if it's 10, 100, or 500 miles. You tire will still leak slowly, and maybe your alignment will be thrown slightly off, but after 100 miles, runflats are 100% toast. So spend the money you'd spend on a can of fix-a-flat and a portable air compressor. Good enough, right?

Having said all of that, I don't believe fix-a-flat is a suitable replacement for a (compact or full-size) spare, but unless you want to carry a donut around in the back seat, fix-a-flat is, again, good enough.

Final disclaimer: Your car, your money, do what you want, eg. my mother doesn't take my advice because she can't change a tire.


  [1]: https://mechanics.stackexchange.com/questions/18276/tire-vibration-from-right-rear-tire-at-high-speeds/18287#18287