As mentioned before, there's plenty of Aperture integration with this release. As well as being able to import photos from (and export photos to) Aperture, Hydra is now available as an Aperture plugin (beta) - based on the all-new Aperture 2.1 Plugin SDK - that allows you to create HDR photos from within Aperture.
Unlike the cross-platform Photomatix Pro, Hydra's UI is the epitome of OS X slickness -- replicating much of the look and feel of both Aperture and other iLife applications. We'll be taking a look at both Hydra and Photomatix Pro in the near future to see whether Hydra's charms are merely skin deep.
Hydra 1.5 costs $59.95, and is a free update for those we already have a license.