Write combining makes video updates a little faster and it's quite effective for 2D work but it also makes things jerkier (specially for games with scroll and or heavy 3D complexity). According to some tests performed by me, it seems Direct3D can benefit a little more than OpenGL and that when increasing resolution and making use of more demanding 3D games, performance tends to suffer more, if WC is enabled. Also, it introduces some lag (write combine buffer must be full before flushing it) and it tends to hog the bus a little, specially when it's operating with heavy loads of data.
The Write combine mode is also less backwards compatible than it's normal counterpart (uncached). Some cards and/or driver combinations can be somewhat picky when write combining is enabled.
In my humble opinion I would have left that option disabled by default.
I have even read on some Ati forums that Ati advises to have it disabled to avoid possible issues, like the infamous "infinite loop".
Regards.
Intel i7 5960X
Gigabye GA-X99-Gaming 5
8 GB DDR4 (2100)
8 GB GeForce GTX 1070 G1 Gaming (Gigabyte)