i7 5775c gets routinely dismissed mostly because synthetic benchmarks don't show any improvement over 4790k, it cannot be overclocked to the same degree as the latter, and on the used market it will set you back $250 and up - pretty expensive as by comparison you can buy a much newer CPU. But the route of newer CPU also requires a new motherboard and DDR4 ram (and a new Windows install, possibly new W10 license if yours is OEM). All perfectly fine if you are ready for a new system which will be much faster overall, and have a path to newer, faster CPUs.
If you are able to overclock 5775c to 4.2 to 4.3 Ghz, and your objective is decent frame rates in games - it is a viable upgrade. Disable the internal GPU and that 128MB L4 cache which does not do much for synthetic benchmarks will do wonders for in-game performance. I'd say in games you would have slightly better than stock i7 7700k performance (still though only 4 cores/8 threads). If you need your system to last another couple of years, it might be a good-enough upgrade.