I noticed a problem with the 2015 update. I often click somewhere in the track to make a 1-click adjustment to exposure, vibrance, etc. This is now inconsistent with the most recent update. Sometimes it updates the appearance in the image to reflect the adjustment, and sometimes it doesn't. However, if you click on the slider knob and drag it, it seems to update the image in real time. There does not seem to be a difference in the behavior whether you have GPU turned on or off in Preferences. Also, if you type in a specific numerical value in the slide box and hit Enter, it always updates the appearance.
Frequently when clicking in the slider track it doesn't seem to recognize the action. It moves the slider knob to that location, but doesn't change the image. If you do a Cmd-Z Undo, it undo's the previous change and not the slider adjustment, so it's clearly not recognizing the action even though it moved the knob to a new position.
I much prefer the 1-click behavior, as it's quicker than clicking on the knob and dragging it. But the 1-click behavior only works intermittently now, which is infuriating. I'm using CC 2015 on a Macbook Pro (Late 2013) with NVIDIA GeForce GT 750M 2Gb running Mac OS 10.9.5.
Adobe has acknowledged this bug in the official Adobe feedback forum: Lightroom CC: Sliders Not Responding after 2015.1 update. Please add your vote, details, and opinion to that topic to make it more likely that Adobe will prioritize a fix.