This thread has made me think about the impact which carrier-bag charging has had on my behaviour, and some other things:
For food shopping I think I was already very good at reusing bags. I would only take a single-use bag if I was buying something like raw meat or cleaning products which I didn't want to put in with the rest of my food. The bags I took for this would then be used as rubbish bags at home, so there'd be no net increase in plastic use compared to buying actual rubbish bags. The big downside I've found is that the clubcard/nectar points that were offered as a "carrot" to encourage bag re-use are no longer offered now that there's the "stick" of the 5p charge.
For non-food shopping I was not previously good at reusing bags - for some things I now am. However I'm still not good when buying certain things, as I don't want to risk putting them in a bag which has been used before and may be slightly dirty - so still sometimes find myself paying. I really should look for some machine-washable plain white cotton or linen carriers.
There can also be some individual cases where the charge increases the damage to the environment: As I work in the IT department of a (non-food) retailer, I spent some time in my local (Edinburgh) selling branch in the run-up to Christmas. One noticeable effect of charging for bags was that customers having paid 5p for a bag would ask for a large bag even if only buying something small, so that if they bought more elsewhere it wouldn't mean paying a second time. This means that extra plastic is being used for larger bags on the off-chance that they'll be needed, rather than only taking the size of bag that'll actually be used. Similarly I wonder if there'll be a long-term shift for certain heavy products (e.g. bottles of juice) towards them coming with extra packaging, carry handles, etc - which may result in a net increase in plastic used.
Overall though, I think the positive effects do outweigh the negative.
