I have the 28-135 IS and it is a good lens.
It is a good lens, but you should be aware if its limitations before purchasing it. Back in the film days, it was a great daylight walk-around lens. I've got one and it still sees some use.
The first limitation is that it isn't very wide when used on a Rebel or 30D class body. With the crop factor, it is a 45mm equivalent lens. You'll find that you need wider in many situations, particularly at WDW. For comparison, at a 20' distance, it can shoot a scene that is 16' wide on a Rebel. With a full frame sensor or film, that would have been close to 26'.
Another issue with it is that it's IS is quite dated. It doesn't have a panning mode. You have to remember to turn it off when used with a tripod or it actually introduces vibration. It's still quite good, but it's definitely their older generation IS.
Another limitation is that it is a slow lens. The minimum aperture of f/5.6 is pretty slow. That will affect your autofocus performance and your low light capability. Of course, getting a wider zoom will cost lots of money and add lots of weight to your kit bag.
As for the actual performance within its capabilities, it is definitely one of the best consumer zooms. The only area where I've have problems is softness when shooting wide angle wide open.
Bottom line - Given that it already substantially overlaps with the range that you have, I think you would be better off with something wider. I'd get a 17-40 f/4.0 if you don't mind spending the money or one of the wider EF-S lenses if you want to keep the budget down. The downside to this approach is that you'll be changing lenses a lot in the middle range.
