Sigma 10-20 QC

How much of an issue is QC with Sigma 10-20 There is £40 difference between in-store and internet pricing. Is it safer to buy in store as it can be changed easily if there is a problem or is it OK to buy on-line and assume there shouldn't be any quality problems. How many people have had problems with sharpness etc with this lens and had to swap it?

Thanks.