The current AMD CPUs are really far behind Intel in performance. The upcoming Zen architecture is supposed to make AMD competitive again, but it's still a month or two away.
That said, for the right workload, an 8-core FX CPU for half the price of a Core i5 can be a decent option. Just be aware that you get what you pay for. The Core i5 doesn't just cost more because of the nice blue sticker.