I use the Hitech soft edge grad ND and hard edge grad ND filters in size 100 mm x 125 mm for more than 3 years now. They are pretty good, especially for this price. I guess the quality of the normal Hitech filters is all the same. Only the size of filters differs. A real difference will only make their new Firecrest glass filter series, which is not yet fully available (only the 4" x 5.65" are for sale).
The Hitech filters are made of very high quality resin, which is normally not an issue. It's light weight and quite scratch resistant. Only when shooting in direct, strong sunlight it becomes obvious that something else was put in front of the lens. Then their might be some additional reflections and a slight decrease in sharpness. But in such a situation lens flare is the biggest concern. Lens flare ruined some of my shots but the reflections of the filter never did. The slight decrease in sharpness is hardly ever noticeable and was no concern for me so far.
For a start I would buy the 3 filter soft grad ND kit. The 0.6 soft grad ND filter will probably be used most of the time. The hard edge grad ND kit is OK but not that useful. I can't remember when I shot the last time with the 0.3 & 0.9 hard grad ND filters.... I guess except for some test shots I never used them again.
Instead of some colored filters I would buy the reverse grad ND filters. Both, the 0.6 & 0.9, are quite useful. I use them very often for sunset and sunrise shots. They really make a difference and are worth the price.
BTW, I never used other grad ND filters before, so I can't compare the Hitech one to other brands.