Regular pool maintenance hasn't been too bad; add some chemicals, flush out the filter occasionally, let the automatic sweep take care of the rest. Having a screen enclosure to keep out tree leaves and debris helps a LOT.
But there will be infrequent large expenses. We had ours resurfaced a few years ago. It has a leak now that we need to get repaired. Had to replace the pump a year of so ago. And so on.
We used it a lot when the kids were young and still living at home, not so much now. It's nice to jump in and cool off after working outdoors, but we're not having the pool parties any more like we did in our younger days.
Be sure to check on insurance. Some companies require you to have child locks on access, some require alarms, some don't allow diving boards or slides, etc.