In terms of performance, FontAgent does not seem to suffer any slow down even if you have a large library of more than a 1,000 fonts. FontAgent also includes many handy tools to manage your fonts in batch. You can quickly clean up fonts and remove errors or even update them. The application also includes Adobe Photoshop CS5 support, so you can quickly upload your custom fonts to an image you're retouching.
FontAgent's strength, the interface, is also its weakness. While it's packed with a lot of very useful features it can be a little confusing to work with, especially for new users. You'll have to give yourself a little bit of time to get used to it.
FontAgent offers a lot of features to quickly manage large font libraries. While a little complex for new users, it comes in really handy once you get used to it.