apple focuses on design. in just this they are lightyears beyond microsoft.
user interface is sleek and extremely user friendly (albeit it might take a couple days to get used to it after switching over). the processor design of the older processors (G5 and before) was optimal for graphics, media, etc. which everyone already knows. of course the intel processors don't take away from this from what i know.
you can still run microsoft office on macs.. although i hate it. office is no good in the first place. but it still works on the mac, with the normal problems it'd face on windows.
you can upgrade hardware... although i dont know the limits on this.
people have already pointed out that plug&play is a dream on the mac. as is software installation. this is all part of the good design of the computers. they make things very simple... and the best part is if you want to do more advanced things on it, you can. OSX is based on a Unix foundation. (so is Linux). you can program on these just like on Linux for the most part.. so you can customize and create your own programs if you want.
i dont know what else to say really. Vista is supposedly terrible. i havent used it's a massive operating system... and i've heard that it's incompatible with a lot of stuff..
on the other hand, Mac OSX Leopard should be out later this year. it basically takes OSX Tiger (current os) to a new level. i've seen some demos and it is amazing i cant wait for it to come out.
oh just some background, i switched to Mac in 05 when i came to grad school. had a 15" PowerBook. it worked great then all of a sudden it started randomly crashing. i didnt know what happened.. so i reinstalled the OS.. but it kept doing it. i was too lazy to take it to the Apple Store so i used it for a couple months like this.. randomly crashing ... but i narrowed it down to when i did certain things. anyway, i finally got sick of it cause i had to reinstall the OS a couple times. finally took it in cause i thought it was a harddrive problem. so they took a look and agreed i had to be a harddrive problem cause everything else looked ok. 20 days later i got it back with a new HD. (oh the 20 days was because one of my keys also fell off and they were replacing the keyboard too... which was on back order). so i get the computer back... first thing i did when i got it was try the programs that had made it crash before.. well.. still crashed. i took it back, obviously wasnt HD, so next i thought memory. long story short.. i showed the guy it still crashed and he said it was a motherboard failure.. he didnt want to make me wait again so he gave me a MacBook Pro. all in all i was very happy.
so to end this long message.. get a mac. in terms of processor speed.. that's not something you really need to base a decision on. there is more than enough processor speed. has been for awhile. the bottleneck is memory. and a note on core duos.. they aren't faster per se. they allow for semi-parallel processing. nobody is going to notice faster speeds unless you are doing hour long or day long computations.