Interesting service, and very well executed.

The one problem that I have with pictograms-as-fonts is anti-aliasing. The designers that I work with spend hours to make their icons absolutely perfect at every pixel size that they'll be displayed at. This is particularly visible when the icon has horizontal or vertical lines. Most of the time you'll want them have a hard edge, and with anti-aliasing you can never be sure you'll get that result.

This will change in a few years when all our screens have high pixel densities, but for the time being I'm sticking with PNG sprites, with hand-tweeked pixels.