Finely, for me, Intrepid is it.
I think I have been testing Linux distros since the first RedHat release. Around 2000, I had a pair of Linux server buzzing quietly around. By 2005 I relied firmly on a Debian distro to provide web services and Ipcop to do the routing but the Desktop battle was far for being won at the time. I kept testing from time to time, Mandrake, then Mandriva, then several flavors of Ubuntu, often on fairly low speed unused machines. Each time, some roadblock made the switch away from Windows just a bit too much of a trouble. By the time I tested Hardy Heron I felt I was getting close. When Intrepid Ibix became available I decided to do a real - make or break - test but I had a mighty incentive to do the switch: the Windows XP MCE edition on my Toshiba P20 notebook had finally managed to screw itself up with no hope of repair: this "OEM only" piece of cr... from MS cannot be repaired, the "reformat the disk, then reinstall-OS-from-provided-CD" option is the only one permitted. Of course I could install a standard XP version but I would have to PAY again for a valid XP (by now you can hear the grinding of my teeth...) and still I would have to reformat the disk anyway.
I installed Intrepid on the P20 and within 30 minutes it was usable. I installed the same Opera that I was using before (9.6) and was able to recover contacts quickly with no fuss. I did spend some time to get the wireless interface working (Atheros) (hint: don't forget to do a lot of restarts every time you change something in the drivers) and that would certainly be beyond the average user capability to get this working but it is not in itself worse than trying to get some drivers to work with Vista. It is part of what a pro is supposed to provide as service when selling a PC+OS kit. After all, not many people install their own OS, Windows or otherwise, so the "difficulties" related to installing an OS should be the province of the provider.
Apart from this episode, the rest of the applications I needed where all supported via the Ubuntu installer except the additional language/thesaurus/hyphen dictionaries. For that I had to use the command line apt-get, something which is not very difficult to use if you see an example and know exactly the package name. Still, it should be done reliably via a graphic interface: 60% of the inhabitants of this planet need to use more than one language to get by (yes, the unilingual countries/regions are a minority) thus such a multi-language functions must be part of the "standard" graphic package installation but I trust it will come soon.
I have been using dual screen Intrepid now for about two weeks and I do like it. I have no regret of Windows Explorer and its sometimes abysmal response time. I think once you get all your mandatory apps up and running, the rest is pretty smooth sailing. Contrary to some opinions expressed on this forum, I do believe that, from now on, Linux on the Desktop is a practical proposition and we should see an increase in use of Linux in 2009 albeit nothing to threaten yet Microsoft OSs. Linux on the Desktop will likely show up in several expended market niches and some new ones. I will continue to have Windows XPs (and 2000!) for some time in my shop, but the replacement of aging equipment will not likely go the Vista route. By 2005, it had become a second nature for us to check that any new app to be installed here had either a Linux version or a Linux based equivalent. The era of Windows-only apps roaming unchallenged is over, not only because of Linux but because several types of services can be provided as well via a browser, rendering the OS irrelevant for the user experience. Of course, we will keep one or two Window machines, along the DOS machine so we can read 5" floppies and QIC tapes ...