Any lens with such a wide focal range will be making a compromise on image quality in exchange for convenience. You would be better off to replace your 55-250mm with any of Canon's 70-200mm lenses. There are many options that would be good replacements for the 18-55mm lens. If you like that focal length, then I recommend saving up for the EF-S 17-55 f/2.8. If you don't need as wide as 18mm, then consider saving up for either the 24-70mm f/2.8L or the 24-105mm f/4L. posted in Anyone used canon lens 18-200mm?
Any lens with such a wide focal range will be making a compromise on image quality in exchange for convenience. You would be better off to replace your 55-250mm with any of Canon's 70-200mm lenses. There are many options that would be good replaceme [