[necron2600]

"not good for anything more than playing around - forget any productivity"

that is quite bleak
I guess it depends on what your productivity needs are. The same thing has been said before about Linux vs Windows.

An SGI, with IRIX+nekoware, can be used today as a systems administration platform and Cloud management (amazon AWS APIs, vmware ESX/vSphere API, etc..), you can also develop with python or ruby, and more, languages. Photography and graphics editing. Vector graphics and painting (with a Wacom pen tablet too). Document and image scanning. PDF creation and manipulation. Desktop publishing (design and publish for digital and print media). Technical documentation creation. Spreadsheets. Data visualization (maple, mathematica, Octave, ParaView, vis5d, CERN Root, etc..). Systems engineering with UML and SysML tools. 3D modeling. CAD design. PCB circuit design. Audio (MIDI (keyboard input, etc..), multichannel XLR mic inputs, editing, etc..). Servers (web, ftp, database, email, file server (smb/nfs), etc..). HD-video editing (albeit not very modern but the basics are all there). Email. Video input (some SGI models) for use with microscope and stereoscope cameras. Some Games. Emulation with MAME, and many classic gaming consoles. Emulation of Commodore64, Amiga, Win95, OS/2, MacOS, AtariST, DOS, ScummVM, DEC VAX and PDP systems (SimH), IBM Mainframes (Hercules), etc...
All that with multi-monitor support too (depending on SGI model).

Of course, there is no modern web browser that runs on IRIX.. but if that is required to be productive today, then a smartphone or chromebook would be all that is needed. I, personally, would simply remote desktop (RDP) into a windows VM to display a modern web browser on an SGI to solve that need.

In conclusion, an SGI system can still be usable and good for productivity today.. depending on your specific needs and expectations.