I've been building PC's since 1995, but I think that the last one that I built was in 2005 with continuous tweaks until last year. When I bought the used 20" iMac 2.4 GHz in 2009, there really wasn't a point in me building another computer. I've since given away the P4 3.2 GHz, and I've sold the iMac and replaced it with a late 2008 MBP 15" 2.53 GHz. Since I'm way past having the latest and greatest PC Hardware like I did back in my teens and 20's, I'm at the point that I buy something used that may a little more than what I need now to carry me 2-4 years. In fact, this MBP that I'm typing on now was an eBay purchase that a crushed display a small dent near the EC32 slot, and I got lucky and found a display for about $300. Overall, I ended up saving about $300 less than what I would pay if I had bought a working one in good condition.
I thought that my PC building days may be dead, but I am still considering building a Windows 7-based HTPC, since all of the cool tuner and cable card hardware seems to be Windows PC only. Sure, there are good tuners and software for Linux and Mac, but the numbers are there for Windows. Unless Apple does something absolutely stupid and MS does something absolutely brilliantly, my personal computer will continue to be Mac. My planned Crashplan and file server is about to be a Mac Mini.