Being a Mac owner/user myself, I'll be the first to say that you really need to know what you're going to do with it before you make a much larger investment on a Mac (yes, Macs are significantly more expensive for somewhat equivalent performance).
Pros:
Superior operating system
No spyware and very little to worry about in the virus dept.*
Stylish design
Good software for editing and media
Built like tanks (really, they are)
Cons:
Expensive initial purchase (plan on spending at least $2500 w/o display for a decent PowerMac)
Difficult to upgrade/modify (even PowerMac towers are)
Expensive parts
Terrible for games (due to lack of available graphics hardware and developer support)
*To elaborate, spyware is essentially non-existent on the Mac (no ActiveX garbage to worry about, as Macs use Apple Safari as their default browser and not Internet Explorer). Viruses are almost never a problem on Macs because Apple has one of the best (if not THE best) software update systems I've seen; and the fact remains, Apple only has like a 5% market share at best. Who wants to write a virus for only 5% of the total PC user base? That's a waste of time. In fact, I've never ONCE had a virus infection in OS X nor have I even thought about installing a 3rd-party virus scanner. Doesn't even seem like I need one.
I'd recommend a Mac if your primary computer use is media and editing (although Windows PCs are really coming very close in capabilities). Other than that, I find them hard to recommend for a variety of reasons. First off, you can get a very fast PC with a good 17" LCD for under $2000, perhaps less. Buy a dual G5 PowerMac with a 17" Apple LCD display and you're talking over $3000. And that's assuming you don't add extras (i.e. a better graphics card, Airport Extreme, etc.). Secondly, upgrading an Apple isn't as easy as going to your favorite computer store and pulling something off the shelf. Lots of Apple machines are very picky with memory timings, requiring you to buy "Apple-compatible" RAM and upgrade graphics cards have to be designed specifically for Apple machines (PC cards, although they use the same bus technology are NOT). Third, if you're a gamer, it's simply not realistic to try to make an Apple game-friendly. Very few software developers think it's worth their time to even port popular PC games to the Mac platform. Although you CAN get an GF6800GT as an option on the PowerMac G5, good luck finding a game for the Mac that will even take advantage of its advanced features.
So, you might ask that amidst all of this negativity why I actually use a Mac. Well's here's why:
1) I don't play games nor do I have time for them anymore.
2) I edit video and layer soundtracks for school - Macs simply have more streamlined software for these tasks.
3) I love the useability and stability of OS X (and I wish Microsoft would get a clue from this).
If it weren't for the combination of those three reasons, I would had just bought another Windows PC.