So, not used the latest Ubuntu then?
As an OS is really is pretty good. Most of the apps are pretty good too - the majority of people can easily use OpenOffice as an Office replacement. There are gaps - a decent video editor that doesn't crash would be good, but these things improve at a rate that would put Windows paid for development to shame.
As to good enough - my AP uses Ubuntu/OO etc and is getting on fine. He doesn't have to worry about viruses, and has all the apps he needs. So, yes, it is good enough, but, and this is the real killer, its getting better all the time.
I use Ubuntu, my other half uses Vista - she is constantly swearing at her almost new machine as it chugs along, constantly dropping network connections, locking up, or slowing to a crawl. I have one to two issues, but I know which I would rather be using.