My S790 arrived this morning. (Also have the S590 and have tested the S390 for over a week too). The S790 is a bit heavier and the construction is certainly improved over the 590/390. Just feels more solid. The illuminated buttons on the top is a nice touch too. The remote control is also improved over the 590/390, with a bit of feedback in the button press.
Other than that ... since I don't own a 4K projector/display everything else appeared to be as the S590. Certainly the IP noise pro setting has more adjustments over the regular one on the S590. The video settings have more customization for BD/DVD/DLNA as well ... the owners manual doesn't exactly make it clear what "direct" and "auto" really do ... so be interested to hear other folks experiences with these settings.
One more thing. THis is a odd one. The SEN portal on the 790 seems to be in full 1080p resolution. With the 590, it appeared to look lower resolution. What does this mean in practice? Absolutely nothing, other than looking visually nicer ! Its the same apps as before .. and those look no different.
First thoughts .... s590 v 790 based on a $100 difference. Tough call. Think I'll need to use the player for a while to note the subtle differences.