From the conclusion:
Despite the price, this lens certainly makes an interesting prospect for those who find the size and weight of a traditional 400mm prime simply too much to be carrying around. The relatively compact dimensions make this lens fairly easy to hand-hold, especially when compared to this lens’ larger and slightly more expensive f/2.8 brother.
Due to its exemplary performance, you’d be hard pushed to not be pleased with this lens. Even though it is expensive, it’s probably worth saving for.