590 user here.
I'm very impressed with the 590. As per the above post, I've never experienced heat issues; and for that matter, I haven't heard of any heat issues with the 590 in reviews or elsewhere.
I switched from AMD to Nvidia for the reason that I was very unhappy with AMD's driver updates, whereas Nvidia is diligent in releasing frequent driver updates to increase performance for brand new games.
The 590 is a generally under-rated card. The #1 complaint I hear is that it's too expensive, and many people say "Buy two 580's instead, it's better way to go!". Well here's the simple answer to that: 2 X 580's costs in the neighbourhood of $1050+. A 590 costs around $750+. Based on any benchmarks, the 590 gives a better price to performance ratio than 2 X 580.
Furthermore, it's the perfect & easiest solution for the normal gamer who want's to be able to max out all graphics in 1920 X 1080, but doesn't want to pay more, and cram two cards into their mid-tower case. Why pay for more than you need? The 590 gives it all with a single card solution.
EDIT: For 3 monitors, you will need at least a 590. If you want top graphics and good FPS, better you do in fact go for SLI.