Depends on which model you want to talk about.So at this stage is the F-15 still "old but good" or just "old"?
Some of the A/B models may be reaching the end of their service lives and are likely not as heavily upgraded.
Then you have things like the Strike Eagles, Stealth Eagles, and the F-15EX, which are kinda old but very useful, a great way to get less radar return on an old air frame, and an attempt to emulate Ace Combat missile loads, respectively.