I have a 27" Late 2015 iMac with a Retina display. I used to work with guidelines that were dots, but I've found that they are too small to see on a Retina display, so practically unusable, making me resort to using lines. However, if I move the same artwork window onto my second monitor (non-Retina), the dotted guidelines return to a reasonable size. If there was an option to increase the size of guideline dots as part of the UI preferences, that would be a great addition. Also: the ability to move guidelines to the back of the artwork needs to be added!