Not all sergers are the same! It is important to review the manual for guidance. But some do a beautiful rolled hem while others just chew up the fabric.
It is important to do research and see what other people have experienced using the serger brand you are thinking of buying.
I have heard several "Bad Comments" about some of the brands and how they can't achieve some of the basic stitches.
Again, ask around and see what other people are experiencing. IT SHOULD BE USER FRIENDLY - first and foremost and easy to thread.