I'd only use it on a image that I planned to print large, larger than 20 X 30. It would be best I think for landscapes or images with lots of fine detail. I wouldn't bother for portraits.It is going to depend on each individual image and the way a user perceives it. I do not believe there is any magical number that works for all.
You're killin' my dreams buddy
So, are there images you shouldn't bother using this tool on? How do you know when going through this step isn't going to result in an appreciable difference?
Should I basically just use it like sharpening? View at 100%, start at 0, go up until I see problems, then back it down? Then recheck at normal viewing size?
Remember, your CR2 files grow to very large size, 3X their normal size, so its not something to use on the 800 images you shot last night.
I tried it a year ago when it first came out, and haven't used it since. I do not take the type of images where I need it.