There was one “feature” in version 1.5.1 that really annoyed me. It was in the export module, it you specified a watermark file Aperture would apply the watermark before it resized the image. So if you were carefull to create a watermark exactly the size you wanted it, it would never actually be that size on the exported image unless you exported your image at full size.
Well, v1.5.2 just got released tonight and they added a “Scale Watermark” checkbox. If you leave that box unchecked your watermark will always be the size you intended it to be, regardless of the exported image size.