This is a great idea, particularly for those of us with double buggies who need 2 hands to at least get them up and down curbs. However, there is one design flaw. If you just attach it and put a water bottle in it it does not hold it vertically - if you had a big water bottle it would tip right out.
So - the way around this is either to always hang a fairly heavy bag on the hook to keep it upright (maybe a handbag if you are beyond change bags) or find some part of the back of the buggy to attach the hook too - this works really well for me.
So it's not a don't buy, but a think about it before you do.