You don't know what you're missing until you get an ultrawide lens. The difference between 10mm and 15mm is huge. Some prefer the Tokina 11-16, but I think the zoom range is a bit restrictive although the f/2.8 is nice. But if you're shooting landscapes, you wouldn't be using f/2.8 much anyway. Indoors, the wider end will get you over 90 degree angle of view, which is the entire room if you stand in the corner.
I have the 17-55 f/2.8 as my general lens, and I got the 10-22 from the canon refurb store when they were running their sale late last year. I plan on selling it when I move to FF, and I see it more as a down payment for the 16-35 II. In the few months that I've had it, I've used it more than expected both indoor and outdoor. I also like that it overlaps my 17-55's range, so that I don't have to switch lenses as often.