Those are my tests. I've also included a shot for a 28-135IS. All at 50mm f/5.6. The crop was choosen in the most difficult lighting area (high contrast) and in the middle bottom of the image.
18-55
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28-135
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50/1.8
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As you can see, the 50 is the best, folowed closely by the 28-135, and the 18-55 way behind. At f/8 the results are pretty similar.
To be fair, the tele end of the 18-55 is the worst part of it. At 28mm it is much close to the 28-135IS and it even outperform it in the borders in CA issues, despite being softer.
I'd like to have the 50mm for some shots I plan to take Monday, but I don't think that's going to happen. 
