Pros:
* inherently more secure. The vast majority of Mac users don't need to use anti-virus software. Practically no viruses in 10 years (thanks to rock solid UNIX under the hood).
* its easier to get stuff done. you won't have to relearn everything every time there is an upgrade.
* Amazing attention to detail. Laptop hard drives that lockdown mid-flight, magnetic power-cord adaptors that don't take the computer with it when the cable is kicked.
* way more durable. Macs are legendary for lasting for years and years while your typical PC is on a continual upgrade cycle from using cheap and nasty components. PCs rarely last more than a couple of years without some important component dying on you.
* they do windows too (natively and virtually).
* greenest computers available. They use materials that are recyclable and re-useable so your grand-kids wont suffer for decisions you make today.
* the best value computer money can buy. The cost of owning a mac over its lifetime is a pittance compared to your average PC.
* they hold their value long after they are designated obsolete.
* it never crashes. Individual programs can die on you, but OS X wont let one program take out another or the entire operating system.
* You only ever see error messages when there is a real problem, and when you do they are in plain english.
* Sleep mode works
* Dead easy to setup and get connected to the internet.
* the Mac applications store (coming in January).
* Less bloat and more efficient use of the hardware.
* bundled apps are full featured and useful
* no trialware, demos or other crud littering the desktop when you first start it up.
* best post purchase support in the business
* Apple is the fastest growing computer maker today. They will be around long after the fat lady has sung and gone home.
* There is only one version of OS X you need to buy, Windows comes in a confusing barrage of different version all with different limitations. OS X upgrades come more often at half the price of Windows ones. Mac upgrades add value to the experience of using your Mac and often make it feel "snappier".
* Aluminium cases won't crack or turn yellow with age.
* Mac laptops are lighter, smaller and easier to lug around with more power to boot.
* target disk mode.
* systemwide searches are faster and can find anything.
* automatic backups that can pull anything from anywhere from anytime.
* better synching with smartphones and mobile devices.
Cons:
* OS X is not windows. Yes you will have to adjust to doing things in different ways, a different interface and design philosophy.
* Apple doesn't really cater for the shoestring budgets.
* You upgrade a Mac every several years (in one hit) instead of incrementally as PCs do.
* You can't legally build your own Mac.
* Industry specific niche software may never become available on the Mac. Counterpoint: but you can run it within Windows or Virtually within OS X.
* computer games come out on Windows first before Mac. Counterpoint: The Mac versions are better tested and include more tweaks. Consoles (Xbox, Playstation, Wii) get games before Windows.
* sticker shock. PC users accustomed to paying a pittance up front get a shock at the initial price of a Mac.
* Apple may not make a Mac specifically for what you want. For instance you can't buy a mini tower Mac or a netbook (counterpoint: MacBook air fills that role better and more powerfully).
* incremental Mac upgrades (graphics cards etc) can be pricey or not even available.
* Apple doesn't support legacy Mac's indefinitely. They continually push the envelope and sometimes they freak everyone out by no long supporting long standing venerable hardware. Apple in previous years has moved on from using floppy drives, hard-drives (slowly moving to SSDs) all kinds of different port types (parallel, serial, firewire 400), CRTs, PowerPC processors, Motorola 68k processors, even CD drives (macbook air).
* you can't buy a Mac with a Blue-Ray drive (counterpoint: but you can get an external plug in one).
* Only one vendor of Macs - Apple. Counterpoint: PCs have very little to differentiate themselves from each other and are in a race to the bottom. Innovation is a dirty word.