I'm not gonna post anything too long right now (have to leave for work soon), but here's my thoughts.
Apple computers in general are well-designed and well-engineered. The components they use (CPU, RAM, HDD, etc) are the same as what might be found in other computers, but they likely get preference for higher quality bins from the OEM's that make them. The battery life on the entire MacBook line is phenomenal, and for the most part, the computers are very powerful for their form factors, considering the cooling required for some of the higher end graphics chips on the 15" MacBooks. You can find more powerful computers for less (sometimes far less if you want to go with Acer), but most will have no access to Thunderbolt (this is changing), and will come in a fully plastic design, which limits durability. Many also won't come with a sudden impact sensor (Toshiba is one brand that does usually have this) to park the hard drive in case of sudden motion.
As for Mac OS X, I'm pretty pleased with the current incarnations as of Snow Leopard and onward. They take a little getting used to at first, but if you've ever used iTunes, you'll pick it up pretty quickly. Coupled with Parallels, there isn't really much that Mac OS X can't do, including gaming (many games are now being ported / bottled with Cider (a WINE fork) for Mac OS). The built-in programs are pretty great, too, and if you have an iDevice, particularly an AppleTV (where you can wirelessly stream your display, another feature that's relatively uncommon still on PC's and requires a separate receiver), there's really no better companion computer for them.
On a budget, though, Apple's hard to justify. That said, the computers retain their value well beyond the purchase date, with some 2007-2008 baseline (13") models still fetching $500 on eBay. Most PC's by comparison tend to lose their value fairly quickly. I've seen at least one person who buys a new set of Macs ever six months, sells the old ones, and basically never actually pays much if anything for a continuous upgrade.