We use a font stack that calls for the most common fonts for windows, mac, *nix and then a generic. The primary reason to use a web font, from what little I know, is to improve (like having some) cross browser, cross OS consistency on how the site renders.
If you look at our site on *nix with chrome its a lot different then doz with firefox they will really will look a lot different. They use different fonts of course, and those fonts are not quite the same in pt size (no matter what our font stack says), so all those em tweaks and line heights layout a bit different.
Instead you can do
@font-face {
font-family: "Open Sans";
font-style: normal;
font-weight: 400;
src: local('Open Sans'), local('OpenSans'), url(http://themes.googleusercontent.com/static/fonts/opensans/v7/cJZKeOuBrn4kERxqtaUH3T8E0i7KZn-EPnyo3HZu7kw.woff) format('woff');
}
Then we update the body stuff as
body {
margin: 0;
padding: 0 0 0 0;
font: 87.5%/150% "Open Sans", Arial, sans-serif;
}
Have to to add bold/italic maybe demi in that as well. Anyway point is at least we would know what we are dealing with. You can get a page delay, mostly if you are using some "off" fonts, but Open Sans is probably already in your browser cache since its very popular.