I think the answer is in what you want to shoot with it and if you want to change your shooting style just a bit. I've used the 10-22 for a wide variety of work and once I came to understand it's strengths and weaknesses I really like it. The wide open feeling you can catch in a landscape is something that you won't feel as much in something with more zoom. It is very easy to distort a photo at the wide end, if you are not shooting level. With landscape work that isn't so much of a problem and you can add emphasis easily with it. My copy is a little soft around the edges wide open, but is very sharp in the center. This is very rarely much of an issue and you can learn to work with it.
Like the others I think the 17-40 is a very fine lens but it would not be wide enough for me. Many times I've been at a waterfall with only one place to shoot from and needing all the width I can get. I have done stitching with other lens in this situation but often the draft from the falls make for a lot of movement in trees around it and make stitching tough. The one shot solution work much better then.
I took the 10-22 to Europe a couple years ago and it was perfect for all those historic buildings and narrow streets. You could never capture as as much in one shot or get the feeling of size like you do with a wide lens.
I have shot the Canon along side friend with other wide lens and it always performs well. While it may not be perfect, at the end of the day, you'll see it really hard to beat. Some of the competition certainly does some things well, but it's very hard to beat it with all things considered. The price has always been a bit steep but very often friends with other wide lens remark to me, "I wish I'd bought the Canon too".
You might want to rent a lens or two you thing you might really like and go shoot them for a bit. I think it wouldn't take long for you to sort out what you value the most and which one would be right for you.
Good luck with the decision,
Dennis
