I have seen a few sites now that use SIFR, and I can see the benefits. I have (quickly) looked into other plugins similar, but SIFR seems to be quite good. So it good in terms of cross-browser compatibility?
It basically runs on everything that has Flash enabled.
On browsers that have flash disabled or no plugin installed a fall back with basic CSS styling can be used.
I use SIFR in a number of sites, CMSs and web apps.
SIFR's fallback behaviour is quite good, so you don't need to worry much about compatibility. It works fine with all major flash-enabled browsers. Due to the replacement of "native" HTML tags it's perfect for SEO, too.
Setting sIFR up is tricky, be prepared for some initial debugging, changing relative paths, and so on. I tend to forget how to set it up every time, it's not very intuitive. But when it works, it works all right. On older machines, you will notice the split second SIFR needs to do the actual replacement. It can be notable and is not nice. On modern, fast machines you will notice nothing, as long as you just replace a few headlines and such. Sifr is not suitable for replacing entire paragraphs of text.
An alternative I have been meaning to check out some time is Cufon. While many people here like it, I can't say much about it myself. It's definitely also worth a look before you commit yourself to a solution.
I was part of a team that used SIFR in the design of a web application in 2008 so we could get a custom font for headlines.
It works OK most of the time but we frequently ran into problems where the SIFR text would be unnaturally wrapped.
That is
A Headline
Would Look
like This
Whenever this happened, we'd have to have our designer look at it and fix it, usually through some fiddling with the CSS.
The other thing I don't like about SIFR is the way the content flashes or blinks as it's replaced. This is really hard to avoid.
All in all, we spent a lot more time fixing SIFR issues that we thought we would, and in the final analysis I don't think it was worth it.
These days, I might look at a CSS-only solution like TypeKit or Kernest.