I'm a big believer in what Mike Mentzer has (had) to say. If you are not familiar with Mike Mentzer I strongly recommend googling his name and perhaps HIT (High Intensity Training). He basically says that you SHOULD go to failure when you work out, but only on one to two sets of lifting each day. In other words, he argues that it is better to spend 10 minutes at the gym doing a lift that completely fatigues and works your body than 2 hours in which you do not push yourself. His argument is that it only takes one or two sets of working to failure to stimulate the growth mechanism in your body, and that countless sets and reps is utterly useless.