The A10-5800K is good only if you're working with a relatively small budget, and only need an entry level platform for your needs. It is cheap because, despite being one of the better APUs available, it still is a fairly low-tier product. It "only" has four cores, half of high-end AMD CPUs, and those cores are paired by their floating point processors. In other words, it actually has two "modules" with each module having two cores. It could also be described (poorly) as a dual-core processor with physical hyper threading.
It trades blows with the FX-4170, and the FX-4300, though both have a slight edge for gaming since they actually have level 3 cache.