They are fast and are great for quickly finishing edges on knits. I use mine to stop the edges from fraying on wovens before I do a prewash, make quick cloth napkins, unpaper towels, dish cloths and putting together knit clothing quickly.
There is a learning curve to them as they are a bit different than a sewing machine and can be a pain to thread. Be careful buying a used serger if you can't test it first. A lot can go wrong with them.