My MBP is nearing 4 years of heavy usage, so I gave it a refresh: Loaded Mavericks, doubled the RAM, and replaced the battery. The difference was immediate. If it starts slowing down again, I'll add a SSD.

Thus far, I've spent roughly $250. Even with the biggest SSD available, I wouldn't even be 1/3 of the way to the cost of a new MBP.

In the bad old days when I worked for a company that still used PCs, I went through a Dell Inspiron every 18 months or so. They started out dog slow and just got worse every day until they eventually just stopped turning on.