As flame angel mention "The quality of this filter set may not be as good & may have colour cast as some bro here has pointed out."
I started out using Tian Ya for a few years, it was ok for normal use but for sunset between 6:15pm to 7pm had very bad color cast (for strong sun light), I shoot in RAW but still found it very difficult to pp away the color cast. As a result many beautiful sunset opportunity was gone due to this reason. I found Hitech GND had less color cast compare to Tian Ya, and during day time it help to produce very beautiful blue sky compare to Tian Ya. But, I still using Tian Ya under rough weather/surrounding condition, ie strong wind with sand/dust.
I am still using three filters holder (Cokin P holder) bought from Tian Ya for the Hitech filter.
Hitech GND filter is about 2 to 3 time the price of Tian Ya's GND, but looking into the image that it produce it worth very cent. I found it most useful for my landscape photographic is the 3 f/stop GND soft and hard, you might want to consider to start with.