Ellipses are necessary but troublesome. Learn to deal with them appropriately.
Català - Castellano
If you have a Boss like mine who loves ellipse shaped Buildings, Drop-offs, Piazzas, etc. you probably have to deal with ellipses more than you wish. Ellipses are quite troublesome.
First of all they don't offset as ellipses, rather when we offset an ellipse the parallel geometry obtained is a SPLINE. Splines have the disadvantage that they can only be trimmed but not extended.

One way to partially solve this problem is to know the PELLIPSE system variable. The default value of this paramter is 0, which means ellipses are drawn as ellipses. If we set it to 1, instead of an ellipse the geometry drawn is a POLYLINE. The advantage of using polylines is that they can be exploded (Ellipses can't) and when offset they remain polylines. Another advantage is that Polylines can be converted to other types of objects (in ACA you can convert them to Walls, Spaces, etc).
The disadvantage of drawing ellipses as Polylines is that since they are an approximation of an ellipse made with multiple curved segments, if we trim part of the ellipse shaped polyline we will probably not going to be able to recover the old geometry by using the extend command.
Another consideration when deciding if you want to set PELLIPSE to 1 or 0 is to think if you will need to modify that ellipse often. If you will be trimming and extending segments of an ellipse my recommendation is to keep it in 0 (the ellipse is a "smart object" you will be able to recover all of its geometry from its smallest segment if is a real ellipse, if it is a polyline yo will have to redraw it) If you will be offsetting the ellipse better set it to 1 so you avoid getting Splines that will leave you with no flexibility.
Another issue I found when dealing with ellipses is when I want to redraw one that is not aligned with the active UCS. For some reason the "UCS Object" feature doesn't allign properly with the axis of ellipses. So, if you want to draw an ellipse using the same alignment for its axis as a previous one this is what you need to do.
You need to know the concept of Quadrant Osnap. In ellipses (and circles) quadrant osnap is a point on the ellipse that intersects the ellipse axis. See the three images below for a clearer explanation (click on the image to enlarge)

The image on the left shows a selected ellipse. The grips we see correspond to the centre and quadrant osnaps of the ellipse that we will use to get the righ UCS alignment. The image in the middle shows how we can access the Osnap contextual menu to select the quadrant osnap. (follow this link for more info on the topic). The image on the right shows the line from quadrant to quadrant drawn. We will use the align UCS to Object to this line to get the correct UCS that will allow us to redraw an ellipse equal or parallel to the original one.(This only works with Ellipses drawn with PELLIPSE = 0).