I find it very accurate if you feed it correct data as said in previous posts.
Probably the hardest bit of data is the scope height because you can't really measure it that easy.
Many people will tell you centre of barrel to centre of scope and even if you could measure that it would be incorrect (granted not by a lot) because what you should be measuring is the scope line i.e. the reticle at the muzzle.
How I do it is measure centre of barrel to centre of scope, zero my rifle very carefully at (for me) 25 yards, when I'm happy with the zero I'll shoot the rifle at 10 yards measure the point of impact and tweak the scope height in chairgun to match it