all great comments - as a part time serger user and former machine sales person - know what you're going to use it for before you get into it. I found that folks who bought one thinking they'd find a use for it usually didn't use it very much. The ones who knew what they needed it for before they bought it were the ones who used them the most.
As snipforfun said, you probably won't get the same accuracy with a serger as you can with a sewing machine, but use of "wooly nylon" thread or similar, can give you nice, soft seam allowances for quilts and garments where you want a different look and feel. I've seen several kids quilts and throws made with sergers with the seam allowances on the outside.