Joaop...If you aren't shooting buildings or other things with straight lines, then the Zeiss 21mm is supposed to be fantastic. I have not tried it, but have tried other Zeiss lenses, such as the 35 and 100 f/2. Be prepared to compensate with a tad of negative exposure comensation, unless you like blown out highlights (you could try HTP, but that's not always the best thing to use). Zeiss glass has a very wide dynamic range. And you probably already know, but the 21mm has "mustache" distortion that is hard or almost impossible to correct. I assume you aren't interested in going wider than 16 or 17mm?
I personally doubt you will prefer the Zeiss 21mm Distagon, over your 17-40L, given the price difference, and the fixed focal length. Sure it will be better, but the results you will get, won't be indicative of the price difference, in my opinion. If the price difference was a bit less, then I would probably say go for it.
If you plan on even doing part of your landscape work in a city, then I would say either the 17 or 24mm tilt-shift lenses would be essential. Also, if you plan on shooting mountains, from a location well below them, in a valley or something...a tilt-shift would come in handy there as well. The same goes for if you are in a high location looking down on something.
You might also consider trying the Rokinon 14mm f/2.8, since the cost is so low. I bought the 85mm 1.4, and it is extremely sharp. Its color and contrast are not "L" quality, but the sharpness is. My copy at least, also has essentially no CA. Of course there is no AF or aperture control via the camera body. I have no idea how the 14mm would compare; I doubt it is as good, and costs more than the 85, but it's a bit less than any other wide angle lens...and certainly a lot less than any f/2.8 wide angle.
Or, if you are open to other longer focal lengths, I can attest that the Zeiss 35mm f/2, is fantastic. I don't know how it compares to the new Sigma 35mm f/1.4 (for the money and the ability to AF, I would certainly buy the Sigma without hesitation...just based on what I have read and seen...and based on my experience with Sigma). But the Zeiss 35mm f/1.4 is supposed to be a tad sharper than the Zeiss f/2. I frankly don't know how you could tell on a (current) Canon full frame body, although you could certainly tell a difference, if there is much of one, on a D800.
I personally plan on getting the Tokina 16-28 f/2.8, because it is optically superior in some aspects, to the 16-35 Canon, yet costs around half the price. I also need f/2.8 for night photography. If I didn't need f/2.8, I would have just been happy with the 17-40L like you have. It is one of the best lens values offered by Canon, or anyone.
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