Cokin is a bit on the cheap side. Colour cast has been noted by many. It's not that Cokin is bad, I have a few like the tobacco grad. But for fine work, I don't think Cokin cuts it.

Hitech are a good middle ground but you have to be careful with them as they can scratch easily. If you are careful with your gear in general, and won;t have them banging about in the bottom of the camera bag uncased, do consider them

I went with Singhray. Not cheap, but quality goods and excellent results. I am in love with the blue-gold polarizer. Lee excellent stuff too but again high price tag.

I have Lee, a bit pricey as already commented but so far I am very pleased with them. I have a wide angle adapter ring, twin holders to allow for rotation and have a set of 3 soft grads (in 0.3, 0.6 and 0.9 densities). My budget didn't stretch to also having a set of hard grads so that's on my wish list for now.

Factors to consider are; budget, lens diameter(s), use of multiple filters including offset rotation, use with circular polarisers, density of ND grads, hard v soft grads. Wisdom seems to be avoid hard grads with higher focal lengths, use either hard or soft with shorter focal lengths depending upon composition.

Also worth noting that ND grads can be used indoors aswell, giving the option to balance out strong window light etc.

I do it in Photoshop with a duplicate layer set to multiply and a mask. You actually have a lot more control over exactly where you want the effect. In the photo below everything except the sky was masked and the blend mode was set to multiply to brng out the detail inthe clouds.

I bought a Cokin set the other day. In Australia virtually a must for daylight.

Cokin is good because it does not matter the lens size, you just get the 77 or 58 mm adaptor.

Also have a Hoya but only 58mm.

The best answer I have found is HDR anyways.