i rode them 2 years ago, weekly after work. In fact, I bought my tw, just to ride these trails. lots of fun, the west side trails were open (not the east side), from 1795 to kelly's pond and beyond.
there is a phone number you can call to see if the trails are open - 1 - 888 361-6908 or 936-344-6205 (this is the one I used). I just called, and all trails are open. Call before you go. the trails close for many reasons, esp after rain. Seems like I need to plan a trip to ride the East trails.
Each trail has its own characteristics. Some sandy, lots of humps, it was slow going. Yes, the south side has more sand and mud than the north side. I had a great time riding them. its quite a work out

. Only real issue is trees down, and you are not supposed to go around them, can't cut them.
The Sam Houston Trails Coalition helps maintain the trails and has maps :
http://samhoustontrails.org/wp-content/uploads/2016/01/map_west.pdf. They do lots of work, esp on the south side, and have work days if you want to help.
I have ridden on all the west side trails.
There are three trailheads, kelly's pond (south), 208 trailhead (middle), and northwest trailhead (north). Kelly's pond is the most busy, and has camping. Be careful during deer season. I like the northwest the most, but it was on my way home, and is the least used. I used to ride the trails till dark, then the service road back to my vehicle.
You need 2 passes, the state OHV (from TPWD?), and the national forest (yearly available from the ranger station, and from nearby dealers) or daily at the trailheads.
To ride on the service roads, you need no passes, but must have a licensed vehicle (they are roads).
I think I averaged about 7 miles per hour when riding on the trails. And I wrecked many times in the deep sand. But had so much fun.
I have since changed jobs, so not up in that area much, but live 45 minutes away, and might be interested in a meetup.
I will try to answer any more question
David