Denine,
While past performance is no promise of future success, I give you the following happy story.
Spike was diagnosed with CRF in January 2001. At that point, a month shy of age fifteen and way down in weight (7 lbs from 11 two months prior), the vet told us to love him and enjoy him. They assumed he had three months, tops. DW and I agreed if we thought him in any pain, as hard as it would be we'd put him down.
Spike has now lived with this disease for over 2.5 years. He'll celebrate his 18th birthday, with any luck, on February 1, 2004. Unforatuntely, he's just been diagnosed with thyroid disease (pretty common in cats this age), and the two diseases are fighting each other hard to make him not eat. The weight is probably going to get him before either disease.
We're sad at this time for fear of losing him. However, we've had almost three years of him enjoying life. We've given him a major run of the house and more love than he knows what to do with. Although I'll be sad when we lose him, I'll know he's been treated like a king for a while and knows how we feel. My suggestions: do your best to get him to eat; don't go bats every time he looks "down," because you'll drive yourself crazy; and spend time being happy you have him, not worrying about when you won't.
Any questions, feel free to PM me.