I have used nothing but windows based computers up until 2 months ago, then my work place switched to mac and I ended up with a macbook pro and it just changed everything. I now have the MBP, the Mac Mini 9 and an iMac
#1 It just worked, no junk to get rid of and it has some phenomenal programs built right into it. I really love ichat for my wife and I to communicate throughout the day via typing, audio or audio/video. Plus we do a lot of file sharing within the office which is ssssoooo easy. Just drag the file to the person you want in ichat and it sends it instantly to them (me and a friend tried a 300mb file the other day and it sent it in under 2 minutes!).
#2 Just like UnaClocker said it really is nice for people that don't really know alot about computers because it's pretty hard to mess it up, but yet easy to mess with for those that do. in iChat there is a screen share feature that I use all of the time. You just pick a person and click on screen share and it will bring up there desktop and you literally have full control of their desktop. It is really awesome and makes helping someone a breeze. Also it turns on their and your mic so you can communicate while on their "computer". A great example is the other day my wife couldn't get anything to print because I haven't set up the printer yet on our iMac, so I just did screen share and set it up for her in about 30 seconds and then she could print. Before the mac I would have had to run home on my lunch break and do it, but using screen share makes everything sssoooo simple.
#3 I know this might sound small and not really matter to alot, but my favorite thing is how quickly start up and shutdown time is on all my macs. On the mini 9 in XP start up time would be about 25-30 seconds and even after that, it still would take another 30-50 seconds to fully load other applications. Like if I clicked on firefox it would give me the hourglass for another 20 seconds or so before actually working. Then shut down was a solid 20 seconds. With os X installed the start up time is right at 15 seconds and is fully functioning immediately without any hesitation or pausing like I experienced with XP. The shutdown time is literally 3-4 seconds! So it makes being on the go so easy.
#4 It works falwlessly on the mini 9. woohoo. I think having a mini 9 is an excellent excuse to try out os X and see if it's for you or not.
I have been to the mac store many times before, but just playing around on the computers for a few minutes, but after having to use one at work on a daily basis for a few weeks then it made me realize the things I like about it.
The only negatives I have is my battery life is about 40 minutes shorter, but other than that there really isn't anything else that I have to complain about. Even though the battery life is shorter though, at least the battery timer is extremely consistent and accurate instead of jumping all over the place like it does in windows.
The only other thing I didn't like using os X was how washed out the screen was originally, but thanks to a few on here that has been fixed and now my screen is extremely vivid and it sounds like the fix is going to be added into the dellefi boot, so I really have nothing negative to say at all.
So anyway, I might be biased now, and I am sure many people have negative points as well, but for me I just made the switch about 2 months ago and I don't regret it one bit and don't plan on turning back any time soon.