I have squatted to failure and I did not like the feel I got for it. The breakdown of form with hundreds of pounds on my back and over my knees and hips does not justify the extra rep it may take to get to failure. I really think hitting failure on compound excercises places too much stress on accessory muscles, they give out first and lead to bad form and joint problems, wherein a isolated excercise the primary muscle hit failure with less structural instability to the joints in motion. Just my 2 cents