They are a huge problem. Because of overfishing, they have replaced the the majority of medium sized fish that prey on baitfish (sardine sized fish). So instead of schools of snapper, we have schools of jellyfish. The last few times I dove (scuba dived) in Florida, the jellyfish were horrible. One dive, which I did without a wetsuit (just swimming trunks and a t-shirt, and my BC), I ran into a group of jellyfish ranging from the size of volleyballs to beach balls. I got the heck out of the water fast. I snapped one photo with my underwater camera, I just couldn't believe what I was seeing.
There have been a few cases were jellyfish have clogged the cooling water intakes to nuclear power plants.
But there is hope. Just like the destruction of California kelp forests that have been reversed by restoring the predators of sea urchins (the sea otter), if we restore the predators of jellyfish, they will decline. Unfortunately, only certain fish and sea turtles eat jellyfish.