I bought a 2013 Mac Pro when it came out, and it's done everything I've asked of it. I deliberately bought it with more resources than I needed, since I figured upgradability would be limited. That was indeed how that worked out.

For better or for worse, very little I do counts on having a decent GPU, but I see real benefits from the 6-core Xeon, and I deeply appreciate how quiet the machine is. Even cranking the CPU for a while, you never hear the fan.

Also, of note, these computers seem to be doing an excellent job of retaining their value. I dug around on eBay and found a machine similar to mine. Mine has 1TB and this auction is for 512GB, but close enough. My machine cost $4319 (academic price, January 2014), and this auction closed at $3451 -- for a three year old machine with lesser specs than mine.