I've asked myself this question numerous times but keep coming back to the idea that the MacPro3,1 (a.k.a. 2008) still has a bit of life left in it.
What is good:
- You can install 32GB of RAM in this thing for approximately $400 (price is average between two options on Amazon and assumes replacing all memory slots).
- You can replace the GPU with something far more modern in comparison to an iMac or "trash can" Mac Pro. I currently have a Radeon HD 6870 installed (PC BIOS, I didn't flash it) and has worked great. You only need to worry about the power connectors. Was an upgrade from the original 8800GT for ~$140 eBay over a year ago.
- The performance of 8 cores makes it still somewhat competitive with hardware from as late as 2013.
- SSD is a cheap way to make it feel like a new machine. Guides are available online on how to homebrew a Fusion drive as well.
- Still supported by OS X, though you might have to replace WiFi/Bluetooth hardware to get the latest features like handoff/continuity (~$40-80).
- In comparison to reports from friends, appears to be more stable than the '09 and '10 models (YMMV).
What is Bad:
- Uses the older 'front side bus' technology Intel chips; access to RAM is much slower than the '09 and later which use the Core i7-class bus.
- Single core performance is much slower than anything made in the last 5 years, even (for the most part) at the low end.
- Only SATA II. As such, your SSD will max out at around 250-300MB/sec instead of SATA III's ~600MB/sec. However, you can work around this with a few hardware dollars.
- Boot camp only officially provides drivers up to Windows 7. Perhaps Windows 8/8.1/10 will work, but I haven't tried it.
- Given it is 7+ years old now, it is a matter of time before it is no longer supported by OSX.
The biggest problem I have is that OSX might stop supporting it soon. But from the technical side, it has 64-bit EFI which is a critical cut-off point for anything past OSX 10.7 Lion. The only reason I could see OSX no longer working on it is because it is no longer compiled for the older Core 2 Duo technology, however, Apple was selling that tech up until early 2012. As such, I would assume support until at least 2016/2017 to give those devices 4-5 years before obsolesence.
So if we assume that OSX 10.11 comes out in late 2015, it would most likely support those early 2012 sold products. As such, Apple will most likely support the release of OSX for 2-3 years before going end-of-life. Assuming 10.10 is the last, you're safe with patches until late 2017 or some time in 2018. If 10.11 supports it, add another year to that. For Apple, supporting an OS X product for 10 years isn't too shabby.
UPDATE: With 10.11 "El Capitan" being announced yesterday with support for the MacPro3,1 I'm guessing it is safe to say that your machine will continue to receive updates through at least 2018.