The Tamron 18-270, a capable "vacation" lens – refrax posted the following feedback on the 18-270mm lens:
Just got the Tamron for a ten day trip to Hawaii coming up next month. The AF performance (outdoors, in good light), sharpness, color, and image stabilization are quite impressive with this lens.
I enjoy the Canon 18-55/55-250 kit combo on my XSi and I plan to keep both lenses, but the ability to mount a single lens that covers such a wide zoom range while on a vacation is very appealing to me.