I use perlite.
Downsides are:
-some people don't like the white look
-sifting it dry gives off a dust that can hurt you if you breathe it in (so don't do that or be really careful with doing that.)
Upsides
-porous
-lightweight
-available
Two cons: It is extremely light and will float out of the pot, even more so than pumice. And aesthetics, that white color. If you're subbing for pumice, the color thing is a wash, so you have to weigh cost to floating.
Personally, I think it's a great soil component if you're using a top dressing.