It does show you what you will more than likely look like post-op. I had a lower jaw advancement on June 4th, 2013 and although my face isn't tense like when I used to posture it forward, it looks pretty darn close. It's an amazing feeling to look in the mirror and see the new me. It's such an improvement and I will never regret this surgery.
On Wednesday I get my bands taken off so I can eat soft food, YES!! I only had one jaw worked on during surgery so the majority of my swelling is down. The right lip and right side of my chin is numb, but bearable.
Anyways, It will help predict what you will look like but beware....you will love your face after surgery!