Beware the Flash Of Unstyled Text! That moment of flicker on your page before your webfonts have downloaded and the screen is showoing your fallback font.
Monica Dinculescu has written a nice tool that shows how your lovely chosen webfont will FOUT (yeah, I just verbed that acronym) against your backup font, but more interestingly, what you can do about it.
Here’s a screenshot comparing Impact and Montserrat:
It’s a really great tool!