I use the HiTech filters for a while now. They do the job but they are definitely not the best you can get. They are rather cheap and you can see why. I've got a full set of grad and reverse grad nd filters as well as the orignal and the new 3.0 ND filter. While the newer 3.0 ND filter is a significant improvement, the strong blue color cast is still bothering me a lot.
If you are really serious into landscape photography, consider buying the Lee filters. The HiTech filters will help to save money but detoriate your image quality.