I have the 936 and LOVE IT! I traded in a 2 year old "jet air" for the 936. Yes converting from cover stitch to rolled hem is not as easy as other machines but I wouldn't give up my 936 for the ease.
I can use any thread and do several design stitchings or anything else I care to do. This is why I did the trade in. Trying to use specialty threads in the "jet air" just was too much of a hassle.
Once you get the hang of it the 936 is a snap to thread correctly on the first shot. I've had mine for a year now and have not had one single problem with it. When I got my 936 I did buy the "work book" The manual is very explanatory and along with the work books you'll be a master as serging in a snap.