This feature is huge IMHO, and I’ve been excitedly waiting for it. I believe it’s the same as copying-and-pasting a long standard prompt every time you initiate a new conversation, but that would hardly be more efficient than saving it once in your profile.

Edit: I just spent the last hour specifying my Custom Instructions and testing them, and yes, this is a killer feature. The responses I’m now able to produce are so much better than before, just by laying out a bunch of general rules and preferences. I find it an enormous relief.