I have noticed that in general my muscular endurance and core strength has improved dramatically. I can push myself and rarely am sore the next day, which was not the case when I first started using the TRX!
I even road 72 miles on my bike (the longest before was 44miles) and felt great the next day! I have been doing some muscular endurance workouts (20 reps) of exercises including the Suspended lunge (10 ground-10 power each leg), Hip Press, Atomic Double Crunch (push-up w/ 2 suspended crunches) , Back Row and Shoulder Super Set (10 Y, 10 T, 10 V), Finishing with Oblique Crunch and V-Ups. Whew! Just completing all reps, regardless of how long it takes I have seen the rest I need in between go down.
Keep varying it up and challenging yourself. Have any good workouts/circuits you do on the TRX that you could share?