I like the software and the OS. Also, I especially like the fact that Apple controls the hardware. Sometimes it means some options aren't available on the Mac side, but it's nice knowing that the OS and Apple software WILL work with your machine. One thing I've noticed is that I spend much less time futzing around with the OS install config in OS X than I do in Windows.
In terms of the actual hardware, I think the iBook is an excellent value. It packs a lot of punch (slot-load optical drive, powered Firewire, 802.11g, Bluetooth 2.0, Radeon 9550) into a small package (4.9 lb), and it has pretty good battery life, all for a very nice price (< US$ 1000).
I think the Mac mini is OK too, but it's not for me. I got the much faster and much more flexible iMac G5. I think the Mac mini is for those short on cash or who don't want to spend too much to try out a Mac. It's also great for grandma, etc. The 32 MB on it really does suck though if you have a 20" screen. Exposé is quite laggy once you have filled up much of your screen with open windows. Not enough memory. (It's not a big deal on the iBook though, since the screen is only 1024x768 anyway and with that screen size people usually don't keep a gazillion windows open.)
However, the iMac is much more of a computer, and it's a pretty good deal too. I have the iMac G5 with 400 GB drive, 128 MB Radeon 9600, 2 GHz G5, and 20" screen. Excellent package. Exposé is a smooth as silk on this 20" screen.