Not sure how much improved the 590 will be over the 660. I had a 660 on my last bike but the guy who bought it paid me extra for it because he liked how it was mounted and that the quick mount was hard wired to the bike. I did not wire the 590 to the bikes speakers to give turn directions via the bikes speakers but instead plan on getting a Bluetooth helmet and have music and turn directions through the helmet speakers. I haven't done this before so I hope I will like it set up that way. The screen does seem brighter in direct sun light but will need to take a trip to see if it's really readable in direct sunlight or if it gets washed out like the 660 did.
My 665 (don't use the XM anymore since I changed the head unit that has XM built in) started freezing a week before my warranty ran out so they just gave me a new one. Before that happened I checked with Garmin to see if the 590 would work with the 665 mount but they said it would not. Probably the thing I liked the best was the "curvy road" option but beside that I really couldn't see that much of a difference especially now that I have a brand new 665 but am definitely interested in what you will think after using..