I have used this with the 7cm extention, it is an excellent gate. My 13-month old has been hanging off this, pulling it, etc and it hasn't moved at all or opened <touch wood>. Fairly easy to fit, it took only ten minutes and I didn't have to drill anything, it holds against the door opening well (it is a concrete opening not a standard wood door frame) - but you'll need a spanner to do it properly, the free one it comes with isn't great.
It has a good mechanism, we have got the hang of it now so it is easy to open with one hand (the trick is to pinch it at the thin pointy side of the opening mechanism rather than the middle!). It is solid, looks nice and to be honest for the price you can't go wrong (I paid £20 on Amazon which I was happy with but see it has now come down in price)
Also remember when you get it out of the box for the first time there is a big gap between the door opening and the side, this is normal, it is not until you fit it that the door locks properly (if that makes any sense at all).