We’re planning on using Cufón to a larger extent than we have before on our build for Gregory, which has to be localized in a number of languages. Using a text replacer for the designed text will allow us to localize text without rendering out new gifs and pngs for every language and whenever copy changes. Not knowing how fast Cufón could render when given a lot of text, I put a test together. PHP renders out a bunch of random text in blocks ranging from 10 characters to 1,000,000 characters. Then Cufón goes to task on each, in sequence. JS tells you how long it takes. Try it out here, but be advised it can crash your browser.
Here are the results of my very non-scientific test:

At 100,000 characters, FF3 threw “A script is running slow, blah blah” messages. IE7 just crashed. Safari 4 hung in there till 1,000,000 but then it gave up. It looks like Apple’s claims about Safari 4’s speed have some truth. I didn’t test any of the past generation of browsers, I may go back and add them later.
It looks like Cufón works perfectly fine on a page where it’s converting a thousand characters or less. But it shouldn’t be used to convert the majority of the body copy of a page. That’s perfectly acceptable to me.