I was at WDW in January to run the Marathon, and I took my old Rebel XT, Sigma 70-200, Sigma 18-50, Canon 50 1.8, and Canon 28-135. The only thing I took in the parks was the 28-135, 50 1.8 and the P-S (Canon A670). Despite some of the average reviews, the lens performed great with the exception of some low light indoor character pictures. THe pictures turned out okay, but if blown up would be a little soft. It would have been great to have this range in 2.8 constant. Also would have liked to have a little wider for the ball at Epcot. The IS delivered some great night pictures of the castle and of the light parade.
Overall, I bought this lens used from the sell forum specifically for this trip, and am definately not dissappointed in the results. I've never used it but maybe should have tried the Sigma 18-200 OS. I never felt that it was too heavy to carry this set-up in a Tamrac Expedition even on the 2 days after running the marathon. I will tell you that Rebel pictures definately look better than the P-S pictures and none of mine have been any bigger than 4x6 either. If I was going back, I'd still take the SLR and a mid range zoom.
Just my opinion though. Different strokes for different folks.