Well first-off let me assure you that I'm not a fanboy of any type. I've used AMD, Intel and Nvidia in many, many builds over many years.
The problem with the FX 9590 is simply that it is a maxed-out, factory-super-overclocked FX-8320. In games it performs about the same as an i5-4690k at stock clocks. What that means is that you can OC an i5-4690k and achieve better gaming performance than the FX-9590 on current games assuming you use a high-end graphics card or multiple graphics cards. Your R9 270x isn't powerful-enough to tell the difference. There are some major disadvantages to the FX-9590:
1) It barely has any room whatsoever to overclock.
2) It requires a high-end cooling solution - AMD recommends liquid cooling (it doesn't come with a cooler). Liquid cooling blocks do nothing to provide airflow to the motherboard's VRM heatsinks - and with a 220w CPU, they are going to need airflow so plan on adding a fan somewhere in there to keep the motherboard cool. It's 220w at max load at stock clocks - that's a LOT of heat to dissipate!
3) It requires an AM3+ board that is compatible with 220w CPUs (not all AM3+ boards can handle 220w CPUs). So even if you find one, I would never use a "cheap motherboard" with a power-monster like that unless you want to risk a fried, dead system.
4) It costs the same as the i5-4690k but REQUIRES a high-end third-party cooling solution and a high-end motherboard. An i5-4690k can get a good OC with a cheap CoolerMaster 212 EVO and a lower-end motherboard (but I always suggest a good, solid motherboard if you OC).
5) There is no upgrade path. If you build with an i5-4690k you can at least drop in an i7-4790k sometime down the road...
6) Since it is such a power-monster you need to make sure your PSU has enough juice.
Obviously, it's totally up to you but I think you'll be better-served in the long run by picking up an i5-4690k and it is the same price as that FX-9590 that you linked. The i5-6600k is also an option but seems to be significantly more expensive. Also keep in mind - if your current system is satisfying your needs, you don't have to change anything.
I've been looking for a direct comparison, but Anandtech's Bench app isn't playing nice (probably user-error heh heh heh) so here's Anandtech's and HardwareCanucks review of the FX-9590. Pay attention to the Gaming benchmarks:
http://www.anandtech.com/show/8316/amds-5-ghz-turbo-cpu-in-retail-the-fx9590-and-asrock-990fx-extreme9-review/8
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/62166-amd-fx-9590-review-piledriver-5ghz-11.html