It looks like a nice machine, but it's pricey. And, I would make sure that the display has on-screen controls - HP's 27 inch ZR2740W display (that will also display billions of colors) doesn't, so it really can't be accurately calibrated. An iMac screen will display millions of colors (not billions) - approximately sRGB - and it doesn't have on-screen controls, so it can't be accurately calibrated. I.E. if the HP doesn't have on-screen controls, then, even though the display may have the potential to display more colors that an iMac, they are in the same class re. color accuracy.
When HP builds a workstation class computer, they do a very good job of it, so I'm guessing that this would be robust as well. But, for the price, you can get a traditional HP workstation, like the Z220 with a Xeon E3-1240V2 (about the same speed as the Xeon 1245) for around $1600.00 US, and can add a non-HP 27 inch AdobeRGB display with on-screen controls that can be calibrated. And, as mentioned by kers above, even though you can add a few things to this All-in-One, with a Z220, you'll have the flexibility of a tower style case, with lots of expandability.
When you compare any workstation class HP to an iMac, the iMac will come up short for professional use - the iMac is designed for consumers, and provides the features that consumers want - including not being able to take it apart, like an extremely clean design (inside and out), and great functionality for home use. But, if you've been a Mac person, I would personally stick with a Mac.