Powerful if you must have free. Slow with Canon 5DS R RAW files (huge) - make sure to use embedded previews if you don't like beach balling and or hanging apps.
If you are simply looking for a tool to rate and review incoming photos before processing, FastRawViewer is inexpensive, powerful, much faster with RAW files and suited to purpose, with a far more Mac-like interface.
Extra star for free and trying to bring the IrfanView to the Mac platform. Would not use in production though.