It is more convenient than searching for change everytime you want to use the MTR but unless you are going to use the system a lot I dont think it is as cost effective as you might think. My wife and I have been twice in the last 6 months as our son is studying there and the last visit we had some friends with us and we just used cash each time.
So in essence it is the convenience factor that you would get it for not any particular cash saving

I can't remember when you were leaving Fiona and I've got a feeling this reply may be too late. But just in case, I'd say absolutely definitely get octopus cards. We've found them invaluable. As well as the MTR, you can also use them on buses, ferries (inc Star Ferry), in lots of shops like 7-11 and Watsons and in vending machines to get drinks.

As bluewes says, it's the sheer convenience of just popping your card into the slot instead of having to scrabble around for change or queue for tickets that makes them so worthwhile.

Elaine

I did use octopus and it was definitely less hassle! Also you can get any unused money on your card refunded- we did this at the airport.

Thanks Fiona, I had been wondering about those. Think we'll invest in one for our trip.