I'm not going to claim they're really cost-competitive with PC equivalents, just that they're fast enough machines to make good workstations, and the price is within the range of what our IT considers a reasonable office workstation purchase price.

At least where I work, desktop machines aren't kept for five years anyway, so I don't think this distinguishes it from any other supplier. Our standard depreciation/replacement cycle for workstations is 3 years, whether it's coming from Apple or Dell. Obviously for home use the economics are different, but then a lot of things are different there, e.g. I build PCs from parts, and there's no way work wants people to be DIY building their own office workstations. And yeah, you can spec out a comparable Dell that's cheaper (though don't forget to add in a high-quality 27" monitor!), but not by enough to really make a dent in the IT budget one way or the other.