Other than Big Bend, you won't find a more challenging ride.  I've done this more time than I can count, though mostly on a DF.  Many of the climbs are named, such as "the elevator shaft".  The roads in this area are nationally recognized as some of the most scenic, and often win awards every year in motorcycle magazines, for the most fun/twisty/hilly roads.  So if it's fun/challenging on a motorcycle, imagine it on a bike.  
 
My only warning is that I rarely had cell service, and the reason there are rest stops every 10 miles or so is that they are not many towns in this part of the hill country.