For walking around and snapping targets of opportunity, I've become very fond of the stabilized kit lens (18-55) that came with my T2i. The combination is super light and I'm not worried about accidents or theft. I had several images published in a travel magazine a few months ago.
The idea of carrying my 5D3 around with a big superzoom makes me feel tired just thinking about it!
A few days ago I tried out the 55-250 stabilized kit lens which I purchased recently for the T2i. Know what? You can take some very nice photos with that puppy if you know what you are doing. I'm thinking about trying both of these plastic kit lenses on my 7D just for fun.
Sorry OP, can't resist answering somebody on 55-250.
You'll not be very disappointed with those plastics though of course their IQ are very far from the IQ of my primes even stopped down. But considering the price and the weight, it is not so bad that a little bit of PP won't be able to make it acceptable.