I have the same setup T3i with the 18-55 kit lens + 55-250mm lens and then I got this Sigma.
- lens is nice quality wise
- 18-250mm is VERY versatile and great carry anywhere
- not too big or heavy
- never have to worry about changing lens in field
- not as sharp as either the 18-55 or 55-250
- Macro is so so. quality.
Considering a lens with this much zoom range it is normal to be "less sharp" as dedicated prime or shorter focal range lens. (just a matter of design and physics with lens).
I actually did 3 or 4 hours reading the web for reviews and comparing it to options from canon and Tamara and this seems to be the best of all three in price and sharpness.
With that said this is a GREAT Price for it. if you are willing to deal with what I mentioned and it's limitation. I keep this on all the time and sold my 18-55 and 55-250mm and don't feel any regret.
I am in the same scenario and thinking about switching out of the 18-55 and 55-250mm for this lens. Can you give me a ballpark figure of how much you gtt for selling your lenses? TIA