Despite having ditched all my Apple gear in the past 12 months and gone back to PC, I think the iMac Pro is a very good product.
I've always been against all in ones and have always preferred a modular machine, partly because I can get better performance out of a modular machine but mainly because I don't want to have to purchase what I will need in 3-4 years today, I prefer to add to it as I go along - which is what most 'workstation' buyers want rather than all-out performance. However I think this product is of high enough performance to question this approach. So long as the RAM is expandable after purchase and the SSD options aren't too expensive I think most people could find a CPU spec that will be good enough for the duration of the machine and add storage externally. This isn't a machine for hard-core gamers, and probably never will be, but for the entry level workstation market, it looks great and the GPU options should be enough for this. HP Z and the like will still take the performance crown because they are simply more expandable, have dual CPU sockets, etc, but we are talking about different categories of product here anyway, the two don't compare and this is where the modular Mac Pro will come in.
I don't think I'm quite an all in one covert quite yet, I'd like to see reviews (especially around noise and thermals) and see what the next Mac Pro is like too. I get many will still prefer a modular machine whether or not they actually need a workstation, I'm not questioning that - if that's what you need, that's what you need. Overall I think Apple have done a good job here at filling the gap until the next Mac Pro comes out, and I'm normally very quick to criticise them. Nice one Apple - now do the same for the MacBook Pro, go build a model that is a step above what you have now please.