If you have kids, then you have to check this place out!  The recreation area has a water park, marina where you can rent paddle boats, stable where you can ride horses, nice parks, great biking and hiking trails.  Right outside the park is the Strategic Air and Space Museum and also the Nebraska Wildlife Safari Park. 
Overall the facilities are pretty good.  I have seen nicer restrooms, but they are equipped with showers and everything you need if you are camping.  Another downside, is the water park is kind of expensive I thought.  If you are just a day visitor, then no big deal, but for those who are camping it would be nice to have a discounted rate at least.
Also, if you are camping, bring some earplugs.  The campgrounds are close to the railroad tracks and you can hear the sound of the train whistle several times throughout the night.  The interstate is also pretty close and if you are a light sleeper the road noise might cuase a problem.  Another option is to rent one of the many cabins that are available which may provide a little more quietness.
I would definitely recommend Mahoney for those with children and who have time to spend to see all the attractions.