As far as I can make out, there are three classes of action that use SEKES. Creating, measuring and transforming/moving. What I find annoying in TC is that the way SEKES work in these categories is inconsistent. For instance, if you are creating, TC does not care whether there are several entities sharing the same SEKE. It will happily snap to a vertex without asking which entities vertex you want to snap to. Not so with dimensions, where TC will ask you over and over which of those identical points you want to snap to. I know there is an option to change the way snaps work in the aperture, but I don't want to be turning that on and off every few seconds.
The I (intersection) SEKE seems to be somehow inferior to the other main ones- it is never offered but has to be chosen. And before anyone says "change the settings", I have!
If I am drawing (as I often do!) a lot of rectangles that are snapped to each other, the magnetic point and auxiliary line feature would be very useful. However, it often doesn't work, although it seems to. You get the auxiliary lines generated, you hover over them, and the thing just won't snap to it. Many times I have drawn a rectangle which I thought was snapped to another feature, only to find that it's missed by a whisker. If you then want to change the size by selecting it, grabbing a side and pulling it to a feature, that's much more reliable.However, magnetic lines don't even appear when you're changing something in this way, so you can't for instance snap a box vertex to the projected intersection of two other entities with confidence.