I have a six-year-old 15" MacBook Pro, which is now on its third battery. A year or so ago the original 120GB hard drive started making ticking sounds, and I replaced it with a 500GB drive (not SSD - at today's prices I would have gone for a 256GB SSD). It's still a perfectly capable machine - a bit constrained by being limited to 3GB of RAM, but otherwise fine.

I had a late 2007 white MacBook whose original battery was still marginally usable last year (the menu status was Replace Soon, rather than Replace Now) - holding about 60% nominal charge, IIRC. The drive had been replace once, following an overheating incident which left it failing.

In the life of my 2008 MBP, I replaced the battery twice and the HDD three times (died mechanical failure once, second time became an SSD).

Maybe more often than you would expect. Don't SSDs have a higher failure rate than their spinning metal counterparts?

at least once?

I have the same laptop and replaced the battery last year and also upgraded to a SSD. This made my old 2ghz C2D just as fast in everyday tasks at the i5 air I got from work.

in other news I just ordered a new MBP Retina to replace it :)

As many times as he wants, he bought a computer.