Most of the time I leave it off since there are times when I shoot that I cannot have a distracting red pattern flash across the subject. I find the AF sensor on the A77 does a pretty good job of locking onto my subject in most situations.
When I do use it, I have found it helpful. It is especially helpful when photographing a subject of low contrast or little to no pattern for the AF sensor to lock on to. Some bad examples (subject choice that is): a flat colored wall with no shadows or texture, a cream colored shower curtain that has texture but no hard lighting or shadows to make the texture pop out enough for a lock on, etc.