You don't have to be a genius to work out that the plot doesn't sound too dissimilar to "The Omen" or "Orphan", it even has Vera Farmiga playing the mother again for gods sake! But, "Joshua" is no where near as great as "The Omen" or "Orphan". Don't get me wrong, it isn't the worst film I've seen in my life and it does have terrific strengths, but also weaknesses that drag the film down quite a bit.
"Joshua" has its own style, it's much more subtle than "Orphan", in "Orphan" we see Esther kill countless people so it's really in your face that she's evil. The twist isn't subtle either. But "Joshua" doesn't make it clear if it's his wrong doing or not. So "Joshua" sets out to be more classy than "Orphan" but also no where near as fun or entertaining. I did like how it was subtle and a bit of a slow-burner but sometimes the pacing got a bit too slow and at times boring, but always engaging.
The most impressive thing about "Joshua" is it's atmosphere. It builds up a really tense and uncomfortable atmosphere. It's beautifully bleak and awkward in a good way, not awkward in a bad way like "The Idiots". The boy who plays Joshua is also quite creepy. The unresolved ending also adds to the films creepyness.
So if you want a quietly grim and creepy drama with splashes of horror and intellect. "Joshua" is for you. But, I prefer the fun and weirdness of "Orphan".