I've been riding the MUTs there for about a year now with a stock Suzuki KQ400.
BE AWARE, the trails can be a bit rough with some of the terrain as its primarily set up for moto and enduro bikes...mind you this is my experience on that stock quad. Regardless its awesome and can be tons of fun just be cool with the moto's around as SOME of them can get a bit..."Karen"...with quad riders sometimes.
Bottom line? If you want mud go to General Sam's. If you want some awesome trails and a place you can have a great look at nature then Sam Houston is the spot.
EDIT TO ADD -
you need an OHV sticker as well as a parking pass. the money goes to taking care of the park.
Activity pass link OHV pass for Texas D