My mother in law bought this for me when my son was born, and it has been a life saver on many occasions. Sometimes I'll need to go grocery shopping alone with the baby, and run into this problem; I only have one pair of hands, so I can't push both a stroller and the grocery cart at the same time.. and using the infant seat (I have the KeyFit 30, which is too big to use in the child seat of the cart) would take up too much room in the grocery cart. For this situation it is perfect. Or if you have no room in the car to take a stroller. There have also been times when there is housework that needs to be done, and the baby just does not want to be put down.
I can be used rear facing or forward facing.. I used it rear facing until my son was a few months old, now I use it forward facing and he loves to be able to look around.
It's generally very comfortable, and very easy to use. However.. when your baby starts to get heavier (around 4 months), it can be a little hard on the back if used for extended periods.. much the same as any other carrier I would think!