I'm using it for the first time this evening (also using Mavericks in anger, for the first time). Couple of issues I've spotted: I applied the polariser brush to an image (the entire image, as I had lots of small highlights) and then reduced its effect to the minimum I needed. This did its job, but but the image (full screen) didn't respond to other sliders I used (contrast, highlights, shadows and so on). No problem, I thought. I've seen similar before, just after a library has upgraded, so I exited Aperture and reloaded. That got me past one problem. The next problem I had was after using a few more functions on the same image. The preview (again, full screen) stayed blurry for longer than I'd expect, and exiting full screen caused the preview in the normal Aperture view to show just part of the image. Again, exiting Aperture and reloading did the trick. Nothing major at this point, and what I've found seems to revolve around the fact I used one of the brushes, but it's not good for my workflow productivity if I have to repeatedly exit Aperture. Hardware? Late 2011 Macbook Air, so still pretty current (and has always run Aperture reasonably well).