I use androidx86 image and Intel's emulator. It works very fast. You can set different screen sizes and some other device features like for normal android emulator.
But there are some cons: no 4.2 version, only 4.0 and 4.1 available. Android image with Google API installed is not available now (you can add them by yourself).
So you can use x86 as major device for developing and testing, but you still need arm android emulator to test on old android versions and you still need several real devices, because emulator is not always correct and there are some test cases you can run only on real device.
P.S. Generally, real device is better (with most popular screen size and android version is much better).