I always seem to work with a lot of knits and stretch fabrics, a serger/overlocker is pretty much essential for that. It's also quick and handy to finish the edges of fabrics that unravel if you even look at them funny, like suede cloth or chifon, so you can prevent unraveling while you're working with them. You can then do a traditional finishing if you'd like to be more professional.
I have a Husqvarna Viking 5-thread overlock. Not bad for a reasonably priced consumer machine. The topstitching sucks. It is only useful for a very few woven fabrics, and is totally useless for anything stretch (which is what it would be most needed for). But the overlocking is pretty good. I'm not crazy about the feed -- it has problems with distorting knits at the ends and I have to do all kinds of tricks to try to prevent that, it also has trouble handling thick fabrics (like where seams overlap). I don't think the 5-thread is worth it, I'd just get the 4-thead standard.
though after cursing with both my consumer serger and my consumer sewing machine, I'd strongly strongly suggest getting an industrial model serger if you plan to continue in cosplaying or some type of costuming. I've found that the kinds of things we create aren't standard, and consumer machines really can't keep up. Most consumer models are meant for things like mom sewing some cotton curtains every now and then. An industrial machine has a stronger motor, works faster, and can perform much better with things like leather, pleather, vinyl, PVC, faux fur, etc. You can push a consumer machine to go pretty far, but after you've been sewing a bit you'll realize it just can't do what costumers need.
a final suggestion: most sewing shops and JoAnns stores offer free or cheap classes to learn how to use a serger. Take one. The directions on my Viking were insufficient, skipping an important step in threading. And the video tutorial that came with the machine was on a VHS tape. Not a DVD, just VHS. I learn pretty well from books, but the class I took was pretty much necessary to learn the machine.