My wife and I took a trip of a lifetime with Paraguay Birding and Nature Tours.  We were most interested in giant mammals and reptiles, but fell in love with birds, too.  The guide is an absolute master of the biology of the Chaco.  We opted for a 5-day package, and it was just four of us total on the trip, with a spotter/driver/cook along.  The vehicle (Nissan Patrol 4x4) was great.  The team was always friendly, helpful, and so good at finding animals.  Lodging was simple in the remote areas, but fully adequate.  The hikes, trails, reserves, and lagunas were spectacular, with so much wildlife all around.  The cook made the best meals for our specific dietary requests.  The guide's understanding of wildlife routines was irreplaceable, as we did pre-dawn patrols, dusk hikes setting up over waterholes to wait into the night, drives at night, and even hikes in the heat of the day for the few species that love that time.  Oscar the main guide is also so enthusiastic and fun.  Overall, very worthy use of our time and worth the (substantial) cost.  Thank you so much to Oscar and Francisco!  Rick and Marta Hintz