This is a good article by Thibaudeau on when to and when NOT to train to fauilure. I pretty much agree with it.
The Thib System — Fatigue and Best Exercises
http://www.t-nation.com/article/bodybui ... _exercises
The nervous system takes as much as five to six times longer than the muscles to recover from an intense session.
So by constantly going to muscle failure, you can overload the CNS so much that it becomes impossible to train with a high frequency.
Taking a set to the point of muscle failure ensures that this set was as productive as it can be.
Remember, simply recruiting a motor unit doesn't mean that it's been stimulated.
You should do both!
In fact, going to failure or not should be an exercise-dependant variable. The more demanding an exercise is on the CNS, the farther away from failure you should stop the set. However, in exercises where the CNS is less involved, you should go to failure and possibly beyond.
When to STOP and NOT STOP.
1. Olympic lifts, ballistic exercises, speed lifts with 45-55% of maximum, plyometrics, and jumps and bounds
STOP When the speed of movement decreases.
2. Deadlifts (and variations), goodmornings (and variations), squats (and variations), lunges and step-ups, free-weight pressing (overhead, incline, flat, decline, and dips), and free-weight/cable pulling (vertical and horizontal)
STOP: One to two reps short of failure.
3. Machine pressing and pulling, chest isolation work, quadriceps isolation work, hamstrings isolation work, lower back isolation work, and abdominal work
Go to failure on at least one set per exercise; you can go to failure on all sets.
4. Biceps isolation work, triceps isolation work, traps isolation work, calves isolation work, and forearms isolation work Very low Go to failure on all sets.
You can go past the point of failure
(drop sets, rest/pause, etc.) on one to two sets per exercise.