It depends mainly on the price and intended use. I still run one of my machines on E5-1620 v2. It can be easily beaten by the new CPUs out there, but at the same time, with a decent GPU, it will allow you to play reasonably well on 1080p, maybe even 1440 (depending on the game).
if you are on a tight budget and the Z420 comes cheap, it could be a reasonable choice. I myself bought few older Lenovo ThinkStation D30/S30 workstations and made them a decent gaming machines by simply upgrading a GPU