Only a handful of software that isn’t a game could end up on the “fun to use” list. I think Windows 7 is on it. I’ve been using it as the main OS on an increasing amount of computers since the pre-release of the public beta (build 7000) since December 08. I’ve now got 5 out of 7 work/home machines using it, most on build 7077 (which isn’t perfect). Waiting on RC build this month. It’s so enjoyable to use (compared to XP and Vista) that I have that “can’t wait” feeling for RTM.
From the responsiveness of the interface, awesome boot and return-from-sleep times, to the overall look, the new “best taskbar eva”, to little things like knowing the difference between a “default audio device” (speakers) and your “default communications device” (headset or webcam/mic)…. it’s got a lot going for it. I’m starting to get more comfortable with the “library” concepts (old 
habits of caring exactly where your file is located on disk die hard). Though, I’m still not a huge IE8 fan compared to Firefox (and now my new fav: Chrome… yes I said it. Just give it’s minimalism a week and you won’t miss the bloat of Firefox’s add-ins).
Just tried the “play to” option in Media Player to push music from my office to the living room PC, which is plugged into the house audio receiver. Couldn’t be easier. Love the simple and effective management window.
Next is to decide how I will implement a HomeGroup, which removes the need for me to manage share permissions, user passwords, etc. on the various home computers.