Jibs are good for those shots that need to go really high. A very good tool for narrative work, but far less useful in other areas of filmmaking. If it's within your budget, adding one to your kit is really useful. And lucky for all of us there's no big confusion, the Kovacam is a huge step above any other jib I have seen. It is very stable, fast to set up (after the first time) and it has almost no carrying weight, especially with the fill-able water weight. it's a great addition to a creative filmmaker's kit.
If you're working on a lower budget or have a restricted carrying limit, just stick to a good slider for now.