Mark Wilson at Gizmodo:

And other than a few specific tasks in which the most expensive Mac Pro’s 8 cores proved beneficial (Handbrake, Cinebench, etc), the iMac outperformed the competition or kept things close enough not to be relevant, plus it straight-up won in the eyes of Speedmark 6.

Performance-wise, the base Mac Pro makes no sense at all. The 8-core Mac Pro offers a touch more power, sometimes, and other times (in many day to day tasks) even it is outgunned.

Of course, any Mac Pro still allows multiple internal hard drives, three PCI slots, more FireWire ports (four vs one) and more room for RAM expansion (32GB vs 16GB). But once again, even in the worlds of professional media creation, that’s a pretty questionable upsell, especially with external storage solutions and the fact that most high, high end media pros (like special effects artists) turn to dedicated render farms to do their heavy number crunching anyway.

With the new iMac, Apple has shrunk the Mac-Pro-needing niche even smaller. And I can’t tell anyone with a straight face that a handful of expandability is worth $300-$1100 with no monitor, no matter how deep their pockets are.