I did it last week. I installed an Ubuntu 6.06 Dapper Drake instance on my main workstation (which is a Fryes GQPC ($170)). I started using it and haven't gone back to the Hoary Hedgehog (Ubuntu 5.04).
To get it all working required setting up some details, things like copying over my book marks from Firefox and address book from Thunderbird.
One thing I had a terrible time with was getting a file manager icon on the top icon bar. On Hoary it was an app called File Manager and it was simple to add. On Dapper it's actually Nautilus and I finally had to create a customer app launcher to put it there. The binary is nautilus.
Another annoyance from this morning is that, when in Firefox and I Save Image..., Firefox insisted on saving a PNG image as a .cgi file. That's a little messed up and different from the behaviour of Firefox on Hoary. I'm sure it's a trivial fix and it might be a decision vs. just an oversight of some sort, but I won't take the time to dig down into what's going on.