I for one am going taking the opposite route. Just purchased a second hand Mac Pro 4.1. If I do the math for me this route seems a lot cheaper. If I max out a iMac 27" on the Dutch Apple Store for a 3,5Ghz 4 core i7, 16gb ram, 1tb fusion drive and GTX780M I come up to € 2296,- ex VAT. For a Geekbench score of 10-14k?
For the Mac Pro route I start off with € 1075,- for my second hand 2x2.26 Ghz 4.1 Mac Pro, 16gb ram and 256gb HD. Add € 120,- for a Samsung 840 EVO 256gb SSD, € 20,- for a OWC SSD mount, € 275,- for a GTX 680 flashed to Mac Edition. Total of € 1490,- ex VAT for a machine with Geekbench of 11-12k. On top of that I just spent € 400,- on a pair of used X5650's on eBay. Add that all up and I come to a total of € 1890,- ex VAT for a Geekbench of 20k+ and future expandability. Also when in 2-3 years the prices of the X5690's drop I can do another upgrade and get even higher Geekbench. In all fairness if you were also to purchase a 27" monitor the iMac is alot cheaper.
But then again, we all have our own priorities. Perhaps some people really need Thunderbolt or like the all-in-one solution of a iMac...I for one am really choked with my Mac Pro and can't wait for my X5650's to arrive 🙂