Not except in specific circumstances, and even in those cases I wouldn't say it hits performance 'like a truck'. And the technical details have already been shared in articles. The only reason I've ever had to mess with defragmentation was a case when Bootcamp couldn't manage to create its partition on a hard drive which had been used for years.
That entirely depends on what you have installed and running and how you use your system. I've been running for almost 3 years on my current MBP and I've never defragged, never repaired permissions, never had any problems with constant beachballing, never had a kernel panic, never had to reinstall an app, and my system runs just as fast and problem-free as when I took it out of the box. I use Safari and frequently have 20+ tabs open simultaneously, in addition to Mail, iCal, Address Book, iTunes, TeamViewer, Skype, Preview, TextEdit, MS Excel and several other apps and widgets open simultaneously. In other words, I use my system a lot, typically 14-18 hours a day.
