I actually don't use my extension table often as I have gotten used to being without one when I go to sew ins or classes.
I have seen those adjustable ones at quilt shows and don't like them. The surface isn't smooth enough for me what with all the edges where the pieces slide to fit your machine. I suggest if you are going to buy an extension table to purchase a one piece one.
There is a plastic company here in Phoenix that will cut whatever shape you want so you can make your own table. The difficulty is getting the legs for the extension table. Anyone who is handy with woodworking type crafts could probably come up with legs for it for you.
At home? Well I just plain got lucky and bought from a dealer a really old cabinet with a hydraulic lift that he adjusted to fit my machine. Then a local store helped make the plastic insert when I got a new machine.
For the ease and money a good cabinet is well worth the investment. I use my machine more when I have ready access to it and can leave it up yet put it away when company comes. I have a really good height adjustable 'chair' which helps.
ali