We have a bouncy castle and it is one of the best things we've ever bought. Hours of entertainment.
Ours is mid-sized with a slide so for ages 4-10 but my parents have a small one for 1-5y and it get used loads as well. Not a fad at all. We have it up all summer, we bring it in at night but it goes up pretty much every day that's dry and not too windy.
We have a trampoline too (both a small toddler one and a big one for older kids) and they get used a bit but not as much as the bouncy castle.
I'm surprised at people saying it's a fad, ours has been a huge success. Mine were not even remotely interested in balance bikes, scooters or their wobble board so guess it just varies between kids.