I had the 10-18 for a couple of years and absolutely loved it. I used it for Architectural interior shots and general 'snap everything in one shot so I can check I'm correct back in the office'. It's super sharp and great fun.
However.... I thought I would use it more for my personal photography and never did. It was one of those lenses I would have in the bag and never take out.
If you know you want it though, it's super well priced on the crop sensor. Maybe try renting one for the day and see just how much you can do with it? Might save you a few hundred quid in the long run!
I ended up selling it together with a couple other lenses with my 50d to bump the price up. The person who bought it uses it all the time! Swings and roundabouts I guess
Oh and additionally, 10mm is much wider than 15mm, it's quite astonishing.