I switched to a Mac Book Pro in late 2008, and I have had no issue with the computer to this day (I am only considering buying a new Mac out of vanity now), whereas the Fujitsu laptop I had before was dying on me, both processor and battery. That particular laptop had the same spec as an Alienware computer of the time, the chassis was the same, motherboard and video card was the same, with the only difference being the display (without a webcam and of lower resolution). My brother bought an Alienware computer around the same time as I bought my current MBP, and his computer did not last more than a year and a half (with the video card dying and needing the "baking trick" to get back to life). He is now in his second Alienware. I have been using my Mac for pretty much everything from work to entertainment, keeping it on and working for at least 12 hours a day.
So, in my experience, if you intend to use the computer heavily and keep it for a long time, I think the investment is worth it (it's much cheaper than two Alienware computers with the same spec). Of course, I have my misgivings about Apple's attitude to overarching control of everything, from content distribution to the use of their hardware, and this might eventually turn me off their products. But then I can install Windows on the computer