Been playing with mine for a bit now and I'm pretty happy with it. The colours are on a par with my previous IPS panel and so far no issues with reponse time or ghosting in the games I've played. I did find it was pretty essential to turn on Vsync as screen tearing is very noticeable on this monitor without it. Not sure if that's due to the size of the panel, refresh rate or the aspect making it worse. Having said that I played counterstrike, Titanfall last night and no problems. Skyrim looks fantastic and that was only running it at 16:9 1440p. Incidentally black levels are great so no issue with any bleed/light in the corners impacting any games you need to run 16:9 with bars at the side. It's very good at that resolution too since it's exactly the same size as a 27" 1440 monitor in 16:9 mode. Need to find a way to get it running at 1440p as I know it can be done looking at videos online.
The only negative I've noticed which has been mentioned is the colour shift when moving from side to side. If you put a single colour background across the entire monitor it's very noticeable that it isn't totally consistent from left to right. Also, whist not enough to be visably noticeable, my stand is about 1cm out from the left and the right side of the montor so there is a very slight lean on it. It's not noticeable but it's disappointing that Samsung don't seem able to consistently keep the monitor level!
As others have mentioned the out of the box brightness is way too severe and I found it better around the 20-30 mark. It's also probably down to me coming from a 1080p monitor but I'm finding the text and some Windows fonts quite small. I might get used to it over time however.
Very happy with it in general though and it's a fantastic replacement for a dual monitor setup. I'm totally sold on the 21:9 3440x1440 aspect ratio and resolution. There really is no going back to 16:9 after trying one and I'm pretty confident we'll see this become a standard as time goes on.