Good.
Do you mean the AVCHD file off the camera or hooking the camera up to your Mac and viewing it. What type of camera are you using, what type of computer do you have and what are the specs, and how are you connecting the camera to the computer? Log and Capture in Final Cut? Import from camera in iMovie? These are things that can change everything. AVCHD files extensions include .mts, .m2ts, and .m2t. These can be converted with a video converter application that supports conversion from the file type you are viewing. Overall, AVCHD does play well on a mac. I think that QuickTime can view it and it will show in HD in QuickTime as long as it hasn't already been compressed. There isn't any reason why the picture would look bad uncompressed, right off the camera. It depends on if the camera can be hooked up as a device through FireWire or USB to view it in QuickTime through Finder.