Do you believe in pet euthanesia and when do you know it is time?

Today I took my 17 year old cat to the vet and his check-up didn't go very well. He has chronic kidney disease and we've known this for a few years now, but he continued to do well. Now it seems to be catching up with him and he has lost a lot of weight and he needed an infusion of fluids today to perk him up a bit.
The vet basically said in a nutshell that cats with his weight loss are nearing the end and we might have a few more months with him, if we are lucky. She said that she predicts that we will have to make the choice soon about whether we want to let him go or not.
I know he is not doing his best right now, but I don't feel like it is yet time. He still has more good days than bad.
How do you feel about pet euthanasia? I know some people think that God or nature should take them and we shouldn't interfere, etc. But I don't know. When do you know it is time?