This lens is fantastic. Everyone with a crop sensor canon body needs one. No excuses.
Pros:
1. Extremely sharp (at least the copy I have is. Seriously, can't notice any difference in sharpness between it and a 70-200 f2.8L IS--it's that good).
2. The IS works extremely well. I've gotten useable handheld shots at 250mm with a 1/15 shutter speed.
3. It's tiny and light.
4. CA is amazingly well controlled and so is distortion. The lens is also sharper at 55mm than the 18-55mm IS.
5. Bokeh is amazing and the lens works surprisingly good as a portrait lens outdoors (you need to zoom way in and be a ways away though).
Cons:
1. Can't use it on a full frame camera
2. Trying to manual focus with it? Good luck.
3. It focuses slowly compared to a 70-200mm L. (However, despite what anyone says,it focuses just as fast as the 70-300mm that has usm, lol. The non-ring usm just can't compare to the real USM on the L lenses).
4. It's fairly slow (f5.6 on the long end).
Everyone with a canon crop sensor body should have one of these. Someday, when I get more cash, I'll pick up a 70-200mm f2.8L IS. However, I'll never get rid of the 55-250mm. It isn't much bigger than the kit lens and weighs NOTHING. I wouldn't go on a long real hike (ie not on a trail) with the 70-200mm f2.8. But you wouldn't want to use the 55-250mm in a low light indoor setting either if you could avoid it.