We had one, a lay z spa Vegas - now have a proper one.
Is it difficult to put up/take down/maintain?
No, easy enough.
Does it require a lot of effort and chemicals to maintain it?
You have to stay on top of the chemicals and get the levels right.
Does it take a lot of time to put it up/take it down?
Takes longer to take down as it's a pain to clean and dry
Do you get the use out of it or does it mostly live in the garage?
We left ours out until it got really cold, then packed it away in the shed. If it's a lay z spa you are looking at, they now do versions that can be left out year long.
How do you empty it and how often must this be done?
If you keep on top of the chemicals the water can last a month or longer. Otherwise it needs emptying every couple of weeks
Having emptied it, how do you ensure that it is dry before packing it away?
The reason we got rid of ours was that we could never get it totally dry. The holes in the walls meant the water never totally came out no matter what we tried so when we got it out of storage, it was often mouldy. Lots of people seemed to have this issue.
How robust are they? Would they take a lot of use by kids, even careful kids, or do they need to be treated very gently?
Ours lasted three seasons before we chucked it as it was looking grubby and tired. It got a couple of punctures during this time from general use. It's fairly tough but if anything pokes it, it seems quite easy to puncture.