Yes, it is almost 10 years old. But the 5K panel in this machine is gorgeous. It's the same panel as LG's Ultrafine 5K, which sells for $1700 and it's also the same panel in Apple's own 5K Studio Display which sells for $2400. Personally I bought one of these converter kits for $375 and use the iMac as an external display, USB hub and charger all through a single USB-C cable.
Alternatively you could upgrade the machine to an SSD and an i7-6700K and it'd be very usable. Just beware the screens are difficult to remove and require a special thin plastic pizza cutter type tool to break the glue. Also, the GPUs in these have been known to fail over time.
EDIT: They sold out! For those asking how to open these up, watch a tutorial. You'll need to purchase this kit https://www.amazon.com.au/iFixit-Adhesive-Strips-Compatible-…
fyi:
27"