With all the news coming out on the iMac Pro, Jason Snell wrote a thoughtful piece over at Macworld about the pluses and minuses of the new machine. If my email inbox is any indication, this question is on a lot of people's mind. Jason's article explains some of the usage cases where the iMac Pro makes the most sense. The article also explains you may want it just because you want the biggest, baddest iMac out there. If that's you, I get that. I've blown money on far dumber things than a kick-ass iMac.
One point I would make, however, is that if you just need raw power and you can afford to wait to see what Apple does with the new Mac Pro (presumably sometime in 2018), you probably should. My guess is that the new Mac Pro will have more processor speed and be more upgradeable than the iMac Pro, but also be more expensive and require you to buy a separate monitor.
As for me, I'm not tempted. My existing 5K iMac is chugging along quite nicely. The hardest thing I make this computer do is render iBooks and screencast videos and the standard iMac is very much up to that task.