Sifr is an interesting idea. However, I decided that the extra time it took to load the page wasn't worth it. I think there was a paper from google that showed that even a tenth of a second difference in load time made people more likely to leave the site.
I have been working with SIFR for a while now and it really is nice if you really want to include every little detail on your website that includes fonts. Choosing the correct font really makes a big deal in web design now and days as before it wasnt as important. Ive known websites that pick very nice and fitting fonts for there designs and it just sets off the design perfectly and in the end it really lets people know that you put work and effort into your website which I personally feel that shows me they are willing to work for what they are trying to provide you with. I enjoy being able to try my best when doing my own web work or helping out clients. SIFR sure is a huge help when it comes to this. It does take a little bit of time to learn and understand it but once you do its a wonderful tool to have.