[Flower]

It really depends. Generally in the first ep yes. To me the OP and ED should be integrally part of an anime series together with the episode contents - it is a whole picture, and sometimes even a commentary, foreshadowing tool, etc when done well.

There are a handful of elements I can think of off hand.

The music itself, audio wise, as a stand alone work - how effective are the music and the lyrics?

The visuals. How well do they compliment or even enlarge on either the audio elements or the story itself?

The overall effect of the combination of both audio and visuals as a stand alone "thing".

The degree to which the op or ed intertwines with individual episodes and the series as a whole.

So anyhoo, different op and ed fall in this spectrum somewhere generally. There are lots of examples of each I could give, but I think the idea is gotten across.

It really does depend on a case by case basis for me, but I generally find it much rarer than otherwise that an op or ed will click with me to encourage me to watch either or both for a series I am following.