I purchased 3 of them many moons ago and they have been running 24x7 since. The only reason I would consider replacing them is if I would require support for drives over 2TB. There have been some posts re failed motherboard, LEDs and power supplies but I suspect that in general these will last a few more years. Over the years I have replaced the smaller drives with the max, 2TB, but other than that I haven't touched them much. I did install FFP. In fact, 2 of the 3 units have been running for just over 300 days since the last reboot. Very stable.
I guess the bottom line is, if it does what you need it to, why replace it?