With two circles, a few lines, and some tangents, watch in amazement as you try to manipulate the larger gear without the smaller gear being locked into position.

Even locking up the smaller gear, in some applications it may still flip flop about the X & Y axis.

If you can get comfortable with this and know what the results can be, you will be better prepared for some of the results you get.  Doesn't really matter if it is 2D or 3D as the results would be similar, just with an added axis.

The more complex an individual sketch is, the more unwieldy it can get.  Keep each as simple as possible.  In that manner you can set the order of precedence more effectively.  Some applications will allow you to set the order within the sketch.  Though naming of dimensions can be achieved, the same can not be said for the constraints.  You can easily find yourself spending half an hour finding an error that would have only taken two minutes to avoid.