For desktops it's either the aging and not user upgradable Mac Pro or the aging (but less so) iMac.
Literally the worst thing I've ever spent 6k on. DO NOT DO IT.
Buy the iMac 5k 3 times instead.
Otherwise you've got to put everything through the external thunderbolt 2 ports. They're fine for storage and there's been some progress on using them for home made gpu expansion boxes but really you'd be better off waiting for Apple to offer something with thunderbolt 3 or internal pci card slots.
I've had a lot of issues with it, specifically:
1. Does not send display signal over mDP consistently. I have 2x 30" Apple Cinema displays connected to it. Sometimes it boots up without displaying anything on them. I have to restart it several times, unplug-and-replug both mDP and USB (the Cinema Display needs USB plugged in to work), clear NVRAM, etc until I finally see the login prompt.
It's not my machine because my partner has the same exact machine with the same configuration and it happens to her.
This is with the D700's so... who knows.
2. USB power is hit and miss. Sometimes it won't power up my Apple keyboard (no key input registered, caps lock light won't turn on, etc). Sometimes it won't power up anything on a specific port/bus until reboot. Again, not machine specific.
3. The SSD (Model: APPLE SSD SM0512F) is slow. I paid $$$ for the 512Gb SSD (the 1Tb was some ridiculous $ at the time, and I thought the better allocation of $ was to the GPU -- I regret this decision now because I'm SOL and can't expand my internal storage. I have a SSD-on-USB3 connected now for my VMs and what not). Sometimes I launch Finder and have to wait like 4-5 seconds before it shows me my home directory. There's really not much in there. A lot of other FS operations take longer than expected.
It's a longer list than this, but I really feel this thing is a lemon.
Maybe they'll address all these issues in the next rev, but my 2008 MP was on its last legs and I needed something to drive two monitors.
The iMac 5k is basically an awesome screen with a free computer attached. You can throw it out (or donate it, or whatever) and buy a new one every 2 years with the money you'd spend on the nMP every 5-6 years. It might not be the most powerful computer in the world, but it's good enough for like 95% of use cases.
I knew Apple was planning on obsoleting it when they broke some USB-2-ethernet software with a update last year but it's lasted longer than any Mac I purchased before and I figure it will be good for another year or two at least.