I have been using these Tommee Tippee nappy bins since I had my eldest nearly 4 years ago and have used it with all my 3 children. I have also tried other nappy bins such as korbel but I would recommend Tommee Tippee as the best one to go for, as it does not smell whereas some nappy bins don't hide the nappy smell that much.
These bins are very convenient, especially if you live in a flat without easy access to the outside bin, or do not have wheely bins in your area etc. To use the nappy wrapper, you place the used nappy into the bin, push it down and then turn to "wrap" the nappy which locks in any smell. This is very easy to do. It is also easy to empty and to replace the cassettes, and can be done very quickly.
I have heard some parents saying they don't find these very useful and prefer just to put nappies into nappy sack and in the kitchen bin but that is hardly very hygenic! and running out to the outside bin every time can be difficult, if you have small toddlers who will want to run out after you then that is not too convenient.
Some people use this without the refill cassettes to save money, but that will negate the whole purpose of the bin so I would not recommend that at all, as it will not lock away the smell without the proper cassettes, so do factor in that cost when choosing to buy it. Overall I would say it is worth it.