My only Apple computer is a Mac Mini, the cheap desktop they do. Bought it when they were using Mac OS 10.4 "Tiger", but have since upgraded to 10.5 "Leopard".
I love Leopard - it's a really good operating system. They finally added some things you'd expect to be there, like little thumbnail images for your photos and videos in Finder (the tool for browsing folders/files, etc.) So it looks nicer that way, and they've put in some transparency and reflections, made toolbar colors and stuff more consistent.
Everything loads really fast and is well organised. You put your favorite applications in the Dock at the bottom of the screen, so they are always one click away. There is one app for settings that covers everything from screensavers to internet, so it's not like Windows with tons of different control panels.
They added a cool thing called Quick Look that lets you instantly open documents, photos, videos without loading an application like QuickTime, and you can even watch video full screen that way, only with basic controls like play and pause. There's a Quick Look button, or you can just tap the space bar when highlighting the file you want to open.
The one crazy thing is not having a full screen or maximize button on apps. There's a button that jumps between preset window sizes instead, so you have to drag on the corner of a window to make it fill the screen if you want it that way. I have a tiny monitor and usually only use one app at a time, so I do like to use all of my screen area. The Dock can be set to auto-hide so it's not always in the way.
The search works but I don't see how you could lose anything unless you have a huge number of files, and all burried in folders. I mean, everything is so well organized, there aren't many places a file could be hiding. A new feature that I don't use lets you set auto-updating search folders. That's not actually the name for it, but I think you could place a folder that contains all files with the word "Joan", and if any new files containing "Joan" get added to your system through email attachments or CDs or whatever, they'll get added to the Joan folder automatically.
Dashboard is a pretty cool app. It contains a bunch of smaller apps or 'widgets' like calculator, weather, calendar. A new thing they added to it is Web Clip, which lets you draw a box around a section of your favorite website and turn that into a widget, so you could grab the top-5 latest Buffy news and put a frame around it and that will stay in Dashboard. You can tap one button to float Dashboard in when you want to check your Buffy news instead of going to the website in a browser. Not very time-savey, because it doesn't take long to go to websites, but it's there. Windows Vista has a similar thing to Dashboard in its Gadgets sidebar.
Oh, a couple of other things to get used to. There's a toolbar at the top of the screen in Mac OS X that stays there. It shows things relevant to the active window, so if you select a Firefox window you'll see Firefox options at the top. This is instead of a toolbar being glued to the top of each window. Also closing a window doesn't mean that application is closed. For example, you could get rid of an iTunes window but still hear music playing. To close iTunes you have to go its icon in the dock and tell it to close.
Don't worry about transferring files over. Compatibility with Windows PCs is very good. And if you transfer something and find you cannot open it because there is no equivalent Mac app, you could always install Windows on the Mac and have both operating systems. Boot Camp lets you install Windows XP SP2 or Windows Vista, and then you hold a key when your Mac boots up to pick between Mac OS X and Windows. Apple made their own update tool that delivers drivers and new versions of their apps like iTunes when you are running in Windows.
