a.) None, unless your site would be useless if the special font were replaced with something generic such as Times New Roman. I don't know how familiar you are with CSS font stacks, but they work just the same with @font-face -attached fonts as they do with regular ones. For example, after attaching a special font with @font-face, I might create a rule similar to the following:
font-family: goose-tracks, microsoft sans serif, arial, sans;
Now if for some reason "Goose Tracks" fails to load, h1 will fail over to the next chosen font. If no backup fonts are specified, it will devolve on the user's default - usually Times New Roman or something similar.
b.) Blow up the user's computer
Only if you use MS Comic Sans and the user is a graphic designer.
(Case in point though - you can't legally embed MS Comic Sans, although most people do have it already.)
Do you have any examples where you have used this with success and that render your pages "graphic designer print quality"??
You can see all the live demos you could ever want on FontSquirrel - go to [this page and click "View @ff demo" just above any font preview. The result, such as [url=http://www.fontsquirrel.com/fontfacedemo/Action-Man]this one](http://www.fontsquirrel.com/fontface), is a live attached font, not an image.
Or is it as simple as if it doesn't work, then a "default" font like Arial will load?
Yes. See above explanation of font stacks.