I've taken some international buses (SJO-MGA), and ones from Rio to Sao Paolo, but usually don't enjoy the long rides that much. It's been more than 30 years since I've been on a Greyhound, and I guess if I had absolutely nothing to do, it might be a good way to see the country.

The unlimited pass sounds interesting. I once won a pass for two on Amtrak for 30 days, but had to work so gave it to a family member to use.