As most landscape photographers know, when shooting just before sunrise or just after sunset and, if we want to include 1/3 of sky in the frame, the part of sky just above the horizon is much brighter than the part of the sky at the top of the frame. Therefore we need these so called reverse nd grad filters. This is where Singh Ray and lately Hitech filters come in to place. They are darker near the lower end of the graduation and they give you a better balanced brightness in the sky, or do they?
Now, this comes from a personal experience and after having paid a very costly $275 + PP + Tax for one Singh-Ray reverse nd graduated filter delivered in the UK. Unless half of your composition includes sky in the frame, the filter simply doesn’t work. As advertised (see pictures on their website), I expected the darker part of the filter on the grad side to be thinner than it actually is (see attachment of the actual filter that was delivered). I tested the filter once and found that it was not useful to me, I was glad I managed to sell it.
So, are the Hitech ones a good alternative? Again, this comes from a personal experience having tried the filter in the field. Not really, they seem to be very similar to Singh-Ray. However, the Singh-Ray one seemed to be neutral where as the Hitech seemed to cause a bit of colour cast during long exposures. You can read a full review of the Hitech here.
So, is there an alternative to the above 2 filters? Well, I use Lee filters and I have adjusted the filter holder to take four filters (four slots). No, Lee don’t make reverse ND Grad filters, yet, but here is what you can do.
I have found that 1 stop on the reverse part of the filter is adequate to cover that bright part of the sky. So, you will need to buy (if you don't already own) 2×1 stop (2x0.3) ND hard grad filters. By inserting them in the holder from opposite sides (see attachment), you create yourself a reverse ND Grad filter. The good part is that you can adjust the size and position of the 'reverse' to precisely suit each scene.