I've been a daily, nightly DOS/Windows user for about 20 years. When OSX 10.0 came out, I was excited (having been forced to use OS9 at work).. until I realised how slow the UI was. When I bought an eMac with 10.3 last year, I was excited, until again, I realised how slow the UI was. This weekend, having decided that I've become too set in my ways, and tempted by the "ooh they're so much faster" lies, I bought an Intel Mac Mini (the fastest one they had), eagerly set it up, and .. it's still soooo sllooooow.
This is mostly because I am overly fussy when it comes to responsiveness. Thanks to my Quake background, moving my mouse cursor around, and typing text in OSX sets off this uneasy "YOU ARE GOING TO GET FRAGGED" feeling that comes from having controls that don't work quite right. I even get more satisfactory results on my Windows machine when it's running as a Synergy client, with the keyboard and mouse plugged into the Mac and everything being sent over the network.
That said, at least the Mac is consistently slow - Windows, in all its blazing 2D-blitting glory, runs crazy fast 98% of the time, and then produces freezing, blank-white-Explorer-windows of frustration the other 2% of the time. I'm also terrified of what might happen with Aero in Vista, whose latest betas run with the same thundering "wait mortal, I am rendering fancy effects" slowness that OSX does. At least you'll have the option of turning Aero off, however.
Still, as Aaron pointed out, I am doing this whole thing because I want differences, and so I should enjoy them.. More detailed discussion on pros (unixness, reliability, ease of use, spotlight, happiness) and cons (single menu bar when multitasking, quartz' font rendering) to come. Maybe.