Looks like a great machine for high-end pro users, including its 1.5 TB max RAM, numerous PCIe slots, and ability to handle four high-TDP GPU's. And I really like the lift-off cover. I don't know if Afterburner offers performance that's difficult to obtain with Windows/NVIDIA workstations, but if it does that's certainly an excellent way for Apple to distinguish its product from the latter.
My one criticism (and this is something I anticipated would be an issue, prior to the announcement) is that most companies that produce workstations for pro users understand that they're not a monolithic user group (i.e., there's a wide range of needs there) and thus, sensibly, produce more than one form factor (essentially, a larger box and a smaller box) to accommodate that range of requirements. Optimally, then, Apple would have offered two form factors as well. Given this machine's high capabilities, an obvious second form factor would be a smaller (but still modular/upgradeable) box that accommodated a maximum of two (instead of four) high-TDP GPU's (this by itself would reduce the TDP requirement by 500W), along with sensible corresponding reductions in other max specs, at a correspondingly lower base price.
And it's not as if Apple doesn't understand this concept. After all, they produce each of the MacBook Pro, iMac, iPad Pro, and iPhone in two (or more) form factors.
Perhaps they'll produce a smaller form factor in the future. I suspect much of the design work (and thus design cost) done for this machine could be used to engineer a smaller box.