My general understanding is that buying a site can be a good way to invest in a money maker if you have cash to spend, but you have to be careful of less-than-honest sellers that will inflate numbers and leave you with a lemon.

What has been your experience?

Good? Bad? Worth the risk?

How easy is it to take a profitable site and add to it to double or triple the revenue?

It all varies really. I have had good, bad and blah experiences.
You just need to verify that all stats are real. It's easy to fake alot of things.
I make sure that anything I buy has been thoroughly vetted.