I was after a good zoom range as a walkabout lens and tried one of the 18-200's and found it soft. Almost as bad as the 75-300 MkIII for IQ at its full extent, but not quite. I ended up getting a Sigma 18-125 OS for a while, which was much sharper and just over half the price. I also tried a Tamron 28-300 VC which was quite good overall but now I'd advise against getting a superzoom as their is always an IQ trade off. I would like to have the Canon 28-300 L IS superzoom though, the IQ of that looks good enough to me, just the price puts me off a bit.
I was left some money a while ago and got a 5DMkII and 24-105 f4 L and recently the 70-200 2.8 IS L so I sold off the lenses I no longer used, among which was the Sigma 18-125, and the Canon pair of 18-55 IS and 55-250 IS went to the wife with my 550D.