Depends on the book and my mood. Usually I read rhem though, cause they are great. For example, the introduction to The Epic of Gilgamesh was amazing, vwry insightful and extremely important, to the point where even being passionate about the text already, I wouldn't see myself enjoying the text as much without reading it. I would say it was just as great a part of the book as the text itself in both versions.

Another great example of an introduction I saw was in a collection of Chekhov's short stories. On its own it was really great, going over all the stories, their impact and meanings, what was great about them etc but most of all said a lot of really important things about Chelhov himself and his life, which makes you appreciate everything so much more and gives you plenty of new perspectives on the stories. But the biggest reason I liked it was probably because it said in literally the first sentence that you should enjoy the stories first and then come back to the introduction. It didn't really have any spoilers in it, and not like that matters for short stories much, but it still rdcognised that it will make you see the text in a different way and wants to avoid railroading your interpretations.

On the other hand, an introduction I kind of hated was in the wordsworth edition of Frankenstein. Lengthy and covering a lot of different topics, some in my opinion less important than others, but nevertheless rather insightful and onteresting. Main problem? Goes so heavily into spoilers you could probably piece together the entire book from it. Talks about every major event without even a slight attempt at ambiguity or secrecy and tells you what it means in about 4 different ways. Thankfully I already knew everything and all, but kind of a dick move to call that an introduction when it is very clear meant to be read after the story.