I have owned the 580s for a while - I picked them up on a whim during one of the occasional 50% off sales on the JBL website. They have mostly been in one of my secondary systems, but they are currently in my primary system. I pair them with an SVS subwoofer and I cross them over at 70hz.
Regarding the 580s v. 590s: I preferred the 580s to the 590s for the reasons others mentioned here: the 590s are huge and they are very top heavy. They were too big for my room. I also think that because of their size, the height of the 590's tweeter may be suboptimal.
The 580s are very enjoyable to listen to. They are not fatiguing and I don't find the tweeter to be too bright (caveat - I am in my 50s). I have them toed in a bit but not pointing directly at me.
I have cycled through so many speakers in my primary system, most recently KEF R11s and Revel F226Bes. Ultimately I realized that I just enjoy listening to the 580s more. I don't know why, they certainly don't measure better or have better components. For whatever reason, in my room, they are terrific, engaging speakers. Hence the promotion to my primary system. Definitely recommended, and the price is right.