Canon EF-S 18-200mm IS vs EF-S 17-55mm IS for Concert Photography – Paul De Bra:

I have used the 18-200IS and the expensive 17-55IS at low light concerts. I find that (in the 18-55 range) the 18-200IS does acceptably well at f/5.6 where the 17-55IS does at least as well at f/4. So the difference in usability for low light is essentially 1 stop. (The 17-55IS is also very good at f/2.8 but that often does not give enough DOF for my applications.)

Excellent Tokina 11-16mm AT-X Pro f/2.8 plus Canon EOS 40D Baby Portraits – All photographed at ISO 1600, and scooterapd says this about the Tokina 11-16 lens:

… some wedding shots from tonight. Simply outstanding results from this lens. Very pleased.

Oh – and if anyone is curious about low light focusing…not a problem. This thing was on-target all night tonight…better than my 24-105 f/4 and on-par with the 17-55.

I got a bouquet toss shot that I could have never nailed with my 17-55, that's for sure.