Using a heat pump for a water heater assist can be a good investment but it also might not, depending on where it is getting the heat from.
If your water heater lives in an un-heated space, it will be significantly less efficient than advertized during winter if the space gets cold (because there isn't much heat to get from air that is 35 degrees).
If your water heater lives in a conditioned space, any heat it gets will come out of the air and it will litterally pump cold air out into the air. This can be helpful in the summer when you are running your air conditioner. It is unhelpful in the winter when you are trying to heat the space with a furnace. In this case, any additional heat will come out of the air which you then have to re-heat with your furnace.
In moderate climates that don't get too cold, heat pump water heaters can be a very good deal. In areas which get cold, they can be a not very good deal (because they cost more than standard water heaters to buy and install).
Unrelated, I've heard they break down more often than standard water heaters and are more expensive to repair because the technitian/plumber needs to be familiar with them. No evidence to back up that statement but just my two cents.