I had to deal with it for work a little bit and maybe my conclusions are useful to others, so here they are [1].

- EUPL is closely modeled after GPL. If you take away one thing then that: If you squint hard enough you will see GPL.

- In a sense it is closer to GPLv3. It has explicit patent related terms. It lacks Tivozation and DRM provisions, so it is not a complete match.

- Beyond what is in the GPL the EUPL tries to be explicit when it comes to license compatibility. EUPL distinguishes between inbound and outbound compatibility and it is very important to be clear when speaking about this.

- Other Copyleft licenses (esp. GPL) are NOT inbound compatible with EUPL.

- GPL (v2 and v3) are outbound compatible besides some other explicitly listed licenses.

The last point is very strange and the biggest quirk in the otherwise pretty boring EUPL. The issue is this: EUPL has some additional restrictions that go beyond GPL. On the other side the GPL does not allow additional restrictions apart from certain precisely defined exceptions ("section 6").

The mechanism the EUPL has chosen to deal with this is that you are legally authorized to switch from EUPL to GPL, and after switching, all the stricter EUPL bits fall away.

[1] I am not a lawyer, consider my vantage being that of an EU startup, also opinions my own, not speaking for my employer.

EDIT: Skimming through the other comments I got the impression that many consider the EUPL closer to the AGPL. The reason I don't see it that way is the mentioned outbound GPL compatibility which renders the additional Affero provisions toothless.

EDIT 2: See bcye's comments which point out an interesting angle that might mean that my views regarding the AGPL issue may be too simplistic.