A couple of months back, I got a good deal on the 16-35 2.8 II. The problem was that I had all crop-factor bodies (7D, 50D, 30D), and the 16-35 just wasn't wide enough for many purposes on a crop body.
Sure, I still have the 10-22 for really wide stuff... but despite how good the 10-22 is, there really wasn't much of a comparison between the two in terms of image quality; at least with my copies of the two, the 16-35 wins, hands-down. Sharper edge-to-edge, better contrast, better color, etc. But it's just not as wide as I sometimes need.
Thus yesterday I treated myself to a 6D as a Black Friday deal ($1400). This despite the fact that I'd promised this would not be a big gear acquisition year for me (as last year I bought the 7D and the 500 f4 II among other things).