I had a pop up ball pool, but now have a more permanent indoor run set up. Pros and cons, as I found them:
BALL POOL
Pros: Easy to set up and take down, inexpensive compared to proper animal runs (mine was £10 from Argos), easy to fold up and store, as the pool has an inner 'lip' it's secure even for climber piggies, inner floor means everything's 'contained' (i.e. no hay, etc kicked out everywhere). Also good as a temporary/ emergency/ quarantine space.
Cons: Need to line the ball pool which can be an issue as it's circular and piggies may burrow/move fleece etc, the floor isn't pee-proof so you do need to be careful about lining/spot cleaning as it's not gonna go in a washing machine! Also piggies need to be lifted in and out, they can't roam from the cage as and when they please (though you can get proper dog/small animal ones that have a side opening).
This is my ball pool:
INDOOR RUN
I made this indoor space using small animal enclosure fencing and lined the floor with a shower curtain with fleece or old duvet covers on top.
Pros: More space, can come and go between the cage as they please (depends on your cage - mine has flip ramps), with the duvet covers I can just pick the whole lot up to change/clean it.
Cons: kinda big and more permanent than the ball pool, not as quick/easy to clear up, need to make sure you've blocked off all escape routes for inquisitive piggies, can't put anything of height (i.e. carrot cottage etc) near the enclosure fencing in case of escapees.
This is my indoor run space: