I like the Dells. But that's about all I've ever worked with, so I'm biased.
On a side note, one of the profs here is ordering a super Mac. He wants a MacPro with dual 6-core Xeon processors (yep, 12 cores), 32GB RAM, a 512GB SSD boot drive, and dual 1TB hard drives. This should be one screaming fast machine (and run hot as a furnace). I may load PS on it just to see how fast it runs.
