My DS6 had 2 years of vision therapy, but he was born with a physical issue that caused the visual processing problems and head tilting. My son was born with visual perceptual deficits and since he is a visual spatial thinker this caused all sorts of problems and delays until he had vision therapy. He had 4 years of pt/ot before we started VT too.

Was it worth it? Without a doubt. But as others have pointed out, it is not for the faint at heart. It's very expensive. VT isn't covered by insurance. The exercises were laborious at times and my son resisted them many times, but then we had to work doubly hard to re-wire his brain and avoid surgery. It takes a lot of patience and effort to pursue vision therapy.

I know vision therapy can vary tremendously by who the behavioral optometrist is. Any behavioral optometrist worth their salt should examine the whole child and assess their hand-eye coordination, motor planning, fine and gross motor skills, speech, sensory issues, and any other existing problems (i.e. head tilting). They should design a program specifically for your child with glasses and daily exercises.

Here's a book on vision therapy that you may find useful. It's a bit technical though.
http://www.amazon.com/Seeing-Through-New-Eyes-Developmental/dp/1843108003

In our case, my son definitely benefitted tremendously from vision therapy. He started vision therapy at 4.5 years old and within a year drawing, reading, and writing and doing math. The developments were staggering, but then my son is possibly eg/pg so the developments started to come fast and thick once the vision therapy kicked in. So yes, vt can make a very big difference.