Once you get comfortable with it, you begin to rely on the thumb rest, so you feel somehow "handicapped" when playing without it. It helps with fast passages and overall playing as well.
The only con is the hardness of the plastic, which tends to leave marks on the body - especially on softer woods or lacquer - of the recorder, when you put the thumb rest on it. That is why I don't rate it with 5 stars.
On the other hand - the good thing is, when you put it on, it stays firmly in place.