Often they say Eclipse sucks, which is completely ignoring the question, and usually doesn't come with any substantial explanation of the supposed suckage. Moreover, talking about how one thing sucks and then silently comparing it to something else implies the other thing has no flaws, which is clearly unfair and stupid.
The only positive non-eclipse-hating arguments I've heard is that it's "easy to use" or "slick", both of which aren't useful to me since I'm familiar and comfortable with Eclipse's user experience. The rare time a feature is mentioned it's something Eclipse has as well.
I've tried to learn it a few times but since no one has been able to articulate any good reason to use it past a kind of anti-eclipse zealotry I haven't spent more than a few hours on it.
I'm not saying it's stupid to like IDEA, I'm saying that I think that preference in this case comes down to wishy washy things like 'feel', which is entirely subjective and entirely inarguable.
I will say this, I've heard plenty of IDEA users be snarky and condescending to people who use Eclipse (I've recently changed jobs and now work in a place seething with them), and I've not once seen it the other way. Anicdata I know, but that is life.