Basically here's what I understand from all the reading and googling pictures. Hope it will help people to decide in which to purchase. But, please correct me if I am wrong.
Belkin / Dr. Bott / Kanex= Simply HDCP pass-through interface for HDCP devices to work on external monitors. Therefore, since iMac, as a monitor, only takes 720P or 2560x1440, so unless these interfaces support up to 2560x1440 and your source is also 2560x1440 (which is unlikely), it will only show 720P on the iMac screen. This results in pixels when one look up close at the iMac screen.
Pros: simple connection, audio get sent to iMac (stereo), space saving, cheaper
Cons: will see pixels when up close since it's 720P on a 2560x1440 display
Close-up actual pictures from the type of devices above:
Atlona AT-HD620 = A HDCP capable interface with scaler built-in. It not only work with HDCP inputs (PS3, BD-players, Xbox360) but also upscale the input signal to the iMac monitor's max resolution (2560x1440). So the iMac sees the input as 2560x1440 while the AT-H620 upscales the input signal so you will see un-pixelated image on the iMac.
Pros: Un-pixelated image on iMac screen, use iMac screen to it's full potential
Cons: device is more bulky, a bit more expensive, (iMac 24" only, audio does not output to the iMac speakers (have to use RCA cables or optical cable to output sound to speakers))
Update:
Correction made by Atlona1: "The AT-HD620 will pass Audio to iMac 27" or 27" LED. The Audio RCA outputs are designed for users of Apple 24" LED displays which d not support audio though MDP connection, only though USB."
Actual pictures from Atlona AT-HD620:
by MacRumors Forum member: hustl3r
Conclusion: If you are looking for convenience go for the Belkin / Dr. Bott / Kanex devices. If you want quality pictures, go for the Atlona AT-HD620.
I am planning to upgrade to a iMac 27" and I will tell you that I'm kinda picky about pixels... so I will go for the Atlona AT-HD620.
Hope this guide help those who can't make up their mind
