I just got the Atlona AT-HD620 as well... here are my impressions.
I got the scaler/convertor box direct from Atlona and a mini DisplayPort cable separately fropm the Apple store. To my surprise, the Apple store's cable was cheaper than the ones at Newegg, etc. I connected my BD player to the scaler box via HDMI, and connected the scaler box to the Mac via mini DP.
When I turned the scaler box on, the Mac automatically detected the new signal and switched over. (The Mac has to already be on for this to happen, and any programs that are running just continue to run. According to the help, you can press Command+F2 to switch between the computer and the external input.)
How does it look and sound?
Video from movies looks great.
Chunky non-antialiased graphics, such as those my cheap disc player creates, don't look as great, because they are scaled up and interpolated. If you back off, they look tolerable. That's not the fault of the HD620, though; it's the fault of my disc player. Here, I think I have been spoiled by the menus and prompts of the Mac's native presentation software, which is of course displayed at full Mac resolution, even if the movie discs the Mac plays natively are DVDs and therefore fuzzier than Blu-ray. The Mac does do a dandy job of interpolating and upsampling standard-definition DVDs though, and of course it can play high-definition video files.
The audio also passes through the HDMI and mini-DisplayPort cables. There is no separate analogue audio connection needed; audio comes out the speakers on the bottom edge of the Mac. There is a pair of stereo RCA jacks on the scaler box for reproducing audio from the HDMI stream if you are sending the video to a different DP monitor that has no built-in speakers.
Pressing Command+F2 does indeed switch between computer display and DisplayPort input display. When you're looking at the computer, the DisplayPort audio is muted. When you're looking at the DisplayPort input, audio from the DisplayPort input is mixed with audio being generated on the computer, such as from iTunes.
While I'm looking at the DisplayPort input video, the computer's infrared port is still active, and I can use the remote to stop iTunes or other player programs if they're still playing.
The ultimate solution, of course, is for Apple to bite the bullet, and put HDMI in and out on the next iMac and include a BD drive and player software. And yes, I know that involves a lot of lawyering up and licensing and protected content paths and whatnot.
All in all, the Atlona does its job quite well. I recommend it.