I absolutely love it! Perfect for my work needs! It's big, but not too big. This 34" 21:9 monitor is very wide (about 31 inches), but vertically it's just 13 inches or so. Just like two monitors put together.
The resolution is 3440x1440, which to me is perfect for this size. The resolution is high enough not to worry about seeing individual pixels from the normal viewing distance, but it is not too high to cause scaling problems for any programs. My laptop has a 3200x1800 resolution and I find it to be too high for a 15" screen. I often have to scale it down to 1600x900 because some programs I use, don't scale too well at Hi-DPI. Shouldn't be a problem on this monitor at all.
The monitor is curved, which I really like. Makes it easier to use it with prescription glasses that have a rather limited depth of field. At the same time the curvature is very gentle and visually does not disturb the image geometry too much (though graphics pros may still prefer a flat monitor).
The U3415W does have HDMI 2.0. But unfortunately I don't think it can accept 4K video signal. I wish it could. 1080p is not a problem of course. I can even watch HD TV in a PiP window in the corner with the sound coming from the built-in speakers. Very cool!
The image is gorgeous! Very sharp and colors look accurate to my eye. This is an IPS display and it comes factory color-calibrated with a Delta E way below 2.0 according to the enclosed calibration report.