Haven't used them, just have seen their ads... I'm sure they are on the high end price wise. I'd shop around and compare... and find some that have specific Pontiac experience.
I found this to be an issue when I had mine done, is that the guy didn't know the Firebirds, and I had to one, point out the things he obviously over looked, and two had to do most of the foot work for him so he would have other quality vehicles to compare mine with, and three, I had to have him redo his evaluation a second time because of the poor research he did. Poor experience to say the least. So shop around and get word of mouth referrals.