Everyone on all the forums I have ever seen say that the Canon refurbished lenses and cameras are excellent. As for the lenses, some people say they are actually better, since a Canon technician has spent a lot of time examining each one and carefully making sure it works perfectly (no front or back focus issues, etc.).
Price: Usually no real savings vs. new retail unless Canon is running a sale.
My suggestion would be to go for it, but to wait until they are running one of their specials of 20% or 30% off. They do that a few times a year. The thing is, the prices on the Canon refurbs are just not all that low normally. At least not compared to online retail prices in the USA (Canada or Europe may have higher retail prices). And since Canon charges you sales tax but other online retailers don't, it means you usually end up paying as much for a canon refurished used lens as you would pay for a new canon lens from an online retailer. But if you wait until one of Canon's 20% off sales, it does sometimes become a decent price discount.
Scotty did a good job to answer you but remember that when these go on sale, due to limited supply and large demand, they go quickly Out of Stock. So be prepare to target what you want and jump on it when the opportunity arrives.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.