I've just taken the plunge & ordered a refurbished Canon EF-S 17-55 f/2.8 lens. Refurbished & on sale, it's still expensive -- about as much as my T1i + other lenses put together! I think my primary usage warrants it, but this still makes me a bit tense.
The Canon warranty on refurbished products is just 90 days. In that time, I'll certainly play with it, try to test sharpness, etc., so I will hopefully feel confident it's not a lemon. But given that this is a small, valuable, highly portable item, is it worth getting a third-party warranty on it?