Right away I noticed that the C920 was a superior product - as it should be, considering it is Logitech's flagship webcam at the moment. But my initial impression was that the extra features might not be worth the size and setup tradeoff. There are three things that the C920 does not have:
1) While all three models attach to your computer monitor in about the same way and all three can tilt up and down, only the C525 and C615 can turn side to side. In fact, they can turn around 360 degrees and more. The C920, however, has a threaded tripod mount, as does the C615.
2) The C920 lacks the excellent "fold-and-go" design of the other two models, which makes packing and travelling easy, while protecting the lenses. In contrast, the C920 is much larger and stationary.
3) The C615 comes with a shorter USB cable AND an extension for more versatility. The C920 - like the C525 - comes with only a full length cable (~6'). My wish for all three models is that they come with a detachable USB cable, which would make them all much more portable and easy to carry around, especially in this age when most people travel with at least one or two USB cables.
But when I actually sat down to do some serious comparisons, I realized just how much more advanced the C920 is, most notably in its use of H.264 compression and its autofocus feature.
H.264 is a standard being utilized more and more for video of all sorts, and it makes a huge difference here