one lizards observations...
The 621 came out a long time ago; it was first.
People had a lot of trouble with it, especially on Ku.
The CK-1 from SatelliteAV and the 741 from WSI/Galaxy came later.
Whether it was better understanding by the dealers & users, or a better product, both seemed superior to the 621, from what I've read.
There were so many people griping about the 621, I never even considered buying one!
I've since spent my money on both the Ck-1 and the 741.
There are several areas of concern for getting the latter two to do well on Ku.
Mostly, it comes down to moving the LNBF to the proper focal point, ...
... AND having the thing centered ON the dish, and pointed AT the center of the dish.
The more careful and detailed users have been able to get pretty good Ku performance.
Those who didn't understand the critical nature of Ku alignment, have struggled.
edit:
I think we have reviews of the Ck-1 and 741, in the Hardware Review department.
There have certainly been some excellent threads by users who installed them, discussing their efforts.
Linuxman and Stogie5150 come to mind right off, but there are many others.