I agree the FX-8350 is a better value and that the FX-9590 is just an OEM overclocked CPU but they are a higher starting point and fun for those who want to play.
Most people don't understand that all CPUs of the same series are the same CPU other than they have been speed binned. That means that the CPU/APU has been tested by AMD and the best reliable frequency at the lowest Vcore is determined and then the CPU is programmed for it's base clock and Vcore. As the CPUs go up in frequency the Vcore tends to increase also.
That being said the FX-9590 is basically the best of the FX model CPUs, the "golden" CPUs that people often refer to when trying to OC a CPU. With a default vcore of 1.5V and a frequency of 4.8 GHz. there is not a lot of room left for OC'ing. The FX-9590 has a TDP of 220w vs. 125w for the FX-8350 - and yet they are both identical architechture CPUs so you can see the increased Vcore and default frequency have increased the TDP dramatically.
Thus if you decide to go with the FX-9000 series CPUs you need to:
1. Be sure that your mobo VRM circuit can handle the electrical needs of these CPUs which only the highend AM3+ mobos are capable of doing such as the Asrock Extreme9 and 990FX Fatality, a few Asus models and one Gigabyte model mobo specifically rated for the FX-9000 series CPUs.
2. You have an extremely capable CPU cooling system - typically a quality H2O system. Most of the Closed Loops Coolers like the Corsair H-series are NOT sufficient to properly cool an FX-9000 series CPU, IME. The Swiftec 220 IS a capable and excellent H2O cooler for the FX-9000 series. The Corsair H100i might work. None of the H-series alone (without the i) are sufficient, IME.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.