I drove my Jeep from Alaska to South America, because I wanted to.
In all, I drove 65,000km (40,000mi) in 22 months, through 17 countries.
Without a doubt, actually leaving was the hardest part.
I highly recommend such a trip.
All my stories and photos are at http://theroadchoseme.com
For anyone wanting to do something similar, http://wikioverland.org has all the logistical information you'll need (borders, paperwork, gas prices, etc. etc.)
Edit: I'm in the early stages of planning the next one, which will be bigger :)