My kit lens showed me I wanted something better and wider, so I was able to pick up a 16-45. I found that I wanted wider and thought that I could compromise on price with the 10-17 FE. I like the 10-17 and the 16-45 a lot. One of the best images I ever took was with the 10-17, however you need to defish if you wanted to move away from having a bend in the image. For 2 years I looked at the 12-24, the price tag, and the images from other posters (while saving up my lunch money) along with the other WA lenses. I almost picked up the Sigma a couple of times - but decided to wait.
I was able to buy the 12-24 a year ago, and its the lens that stays on the camera most of the time. To me it is like having a set of primes, very sharp - I notice very little distortion in it, have had no problems with flare (and I shoot into a lot of sunsets), and its a bit large - but I manage well with it. I use it for just about everything.
In my backpack I have (and most of the time travel with) the 10-17, 12-24, 16-45 and 55-300. That is it (more than enough).
I have been considering a prime - the DA 21/3.2, however with the 12-24 I keep putting it off as not really needed. Right now I have no urgent need for anything else. I feel I have everything covered. The only change I might make is if Pentax were to come out with the equivalent of the Tokina 11-16/f2.8. I could see myself swapping lenses there, however I would loose the 24 end and have to go with the 16-45. I find the 12-24 being better at 24 than the 16-45, but then again it may be my old tired eyes (with coke bottle glasses)....
So, just as an illustration, last week I was back east on business. I was able to spend a couple of hours at the USAF Air Museum at Wright Patterson in Dayton. Here is a shot, has wonderful detail (in full size), handles flare very well, very little edge distortion, great DOF and field of view - excellent color in a very difficult lighting situation (think of a cave with a flashlight). No! it was not handheld - on a tripod for about 6 seconds at f8 - if I remember right.