I am basically addicted to my 15" Powerbook Pro.  It runs on OSX 10.5.8 and it is super reliable, fast, and user friendly.  Before I got my Powerbook Pro I had a G4 (a similar model) and it really stood up to the college-related beating I gave it.  It ran almost perfectly and virus free for four years, despite the fact that I didn't shut it down as often as I should have, dropped it numerous times, used it with 4-10 applications open at all times for 4-10 hours everyday, etc.  So far, my 15" Powerbook Pro seems to be just as reliable and durable.  I've now had it for almost a year and a half...