Have A Bernina Five Thread Serger
Haven't used it as much as one thought one would (purchased to run up laundry and other types of sacks/bags), but it works a treat.
Many vintage and modern sewing machines have serger (more accurately overlock)feet, but the effect varies and IMHO is nothing like a true separate unit.
Depending upon how many threads (two, three or five) a serger will cut and do a three thread overlock stitch. This is a very tight, firm and strong stitch for seams such as found on jeans, cushion covers, and so forth.
Sergers are also good for running up knits and swim wear, anything where you want flat seams.
To get the most use out of serger you have to really become comfortable with the thing.
Many units will run quite fast, much faster than a sewing machine and that scares some sewers. Then there is the dreaded "T" word; threading.
It took me almost an entire day, one bottle of gin, hundreds of swear words, grey hairs and so forth before I finally got my five thread Bernette threaded. Now that it's done whatever is being sewn will used those same colors, PERIOD. Am not going though that again in any hurry.
A good tip I learned rearding threading sergers: when needing to change a color and or at the end of a spool of thread do not unthread the machine.
Instead cut off a good length of the old thread, then tie it with a good firm knot to the new thread and *gently* pull from the needle end. If you are careful and lucky you will be able to swap out the old thread quite easy without having to resort to totally rethreading from scratch.