I bought a serger a bit ago thinking I'd use it when making clothing only to discover that it only does a 2-needle/4-thread 'mock-safety stitch'. It was nice for finishing the edges on a silk liner I made to keep the edges from fraying (instead of using binding tape or folding the fabric in on itself and topstitching, say) but sadly its one seam really hasn't been much use to me.
As I've paid more attention to seams on clothing, I learned that the seam types I really want in a serger are the rolled hem and a flatlock stitch. The rolled hem can be found on t-shirt bottoms. The fabric is rolled under and two needles on the right side are matched with a 'looper' thread or two on the other. Flatlocking is where two fabric edges overlap each other, right side against wrong side (rather than right sides together as usual), and 2-4 needles work with top and bottom looper threads to make a nice flat seam with threads wrapping around the raw edges of fabric to keep it from fraying (something like that anyway, I've only read about it thus far).
These are great seams, but my machine is older and simpler and can't do them. Machines that can seem to be awfully expensive, so it'll be a while before I upgrade to one that can.
In my opinion, sergers are specialized machines and unless you mostly make clothing they may not be worth it. But for $100? Not a bad price for making seams go faster, and as mechanical devices they're intricate and fascinating.