As a result of my QuickCam Software write-up, I've had a couple of exchanges with the Guru from the Logitech User Forum, and one of the Logitech software engineers. I would like to make a couple of things clear...
First, it is important for webcam users to understand that over the years, Logitech has several times released "updated" versions of some cameras which actually have entirely new (different) insides, but look exactly the same and have exactly the same name as the "older" model. As I said, my QuickCam for Notebooks Deluxe was one example of this; my brother's QuickCam Pro 5000 was another. If you look at the Supported QuickCam 11.0 Devices table, you can see a lot of examples of this - just look for multiple lines with the same name and different PIDs (Communicate STX, Orbit MP, Fusion, Pro 5000...). What this means, in almost every case, is that as the cameras age, and they start to be dropped from the supported device list, the "older" model gets dropped first, and the "updated" model continues for a while, which means two seemingly identical cameras in fact have two different "latest driver" versions. Try to use the newer driver with the older camera, and it doesn't recognize it, and thus won't install. Continue to use the old driver with the newer camera, and you are missing out on the latest work done by Logitech engineers - and right now, that is particularly significant, because as I said before, the 11.7 and later drivers have fixes to some rather serious problems with installation, error logging and Video Effects, to name just a few.
Second, I don't mean to criticize Logitech directly for the way the software update procedure is done, or even for the warnings that come up on the software download page. Given the unfortunate situation that I just described, I don't have a better solution. If you have to choose between downloading software which you know will not work for some cameras, or not downloading it and thus leaving some cameras running an older software which could actually be updated, the choice is pretty clear - especially if you are the one who has to provide the technical support when the owners in that first group call and want to know why their camera won't work any more, or the drivers won't install for it.
My recommendation to Logitech would be, don't do that any more! If you are making a new camera, give it a new name, and preferably at least a slightly new look, so people can determine what camera they really have! Even subtle changes can be sufficient - I recall that there was a difference in an earlier model (maybe the original QuickCam Pro?) where one version had a dark-colored focus ring, and one a light-colored focus ring.
My recommendation to Logitech QuickCam owners would be, when you are considering a software update, make sure that you know exactly which camera you have. The Release Notes usually contain a Supported Devices list, referenced by 8-digit part number, (found on the black tag on the camera USB cable) or the USB/PID, found in Windows Device Manager or with the Linux lsusb command.