Like many of the mounds associated with the Mississippi-type mound builder cultures, these are far more subtle than the ruins in the SW (e.g. Mesa Verde). Nonetheless, quite interesting and worth a quick visit. Because of the layout of the trails, one can hike in for the first loop in maybe an hour. If time permits, continue hiking out past more mounds to one of the overlooks of the Mississippi. If you're a park-bagger, it's definitely worth the stop.
We stayed at
Pikes Peak SP just south of McGregor. Now coming from Colorado, it wasn't the Pikes Peak we were expecting ;) but it was named after the same person! Electric with a dump station, showers. Only about 15 minutes from Pike Peak SP to Effigy Mounds.