Cameron Moll has written a very thoughtful and objective article Exploring Cufón, a sIFR alternative for font embedding. Cufón is an SVG (scalable vector graphics, similar to Flash) and Javascript alternative to sIFR, which uses Flash and Javascript.

Pros:
+ No plug-ins required – it can only use features natively supported by the client
+ Compatibility – it works on every major browser on the market
+ Ease of use – no or near-zero configuration needed for standard use cases
+ Speed – it's fast, even for sufficiently large amounts of text

Cons:
_ No text selection. You can’t select the text to do something simple such as copy and paste
_ No hover state. Yet.
_ Older PostScript fonts on Mac OS X aren’t supported. Requires converting font files. Eula alert?
_ Letterform fidelity may be inferior to sIFR. (But if Cameron says it's sufficient for his needs, I'm inclined to listen.
_ Typeface information (files) may be easier to thieve than with sIFR.

It raises some questions. It's a font embedding mechanism, but is it fundamentally different from sIFR as to warrant foundries' revisiting their licensing agreements? If that last point is true, we can expect some foundries to get ruffled.

Worth reading his in depth piece.

It's also worth writing the Typo Wiki entry for Cufon.