How much hot water are you using? How many people in the house?
The problem I see with them, if you have them inside the building envelope (climatized area) your just robbing Peter to pay Paul most of the year. If they are outside the envelope, everything will need to be very well insulated. When you need it the most (winter, when water temp is 40F) they drop in efficiency. If your in an area with high AC needs, you can use the waste heat to heat the water (so you benefit from both sides of the transaction). You move heat from an area you want cold and put it in an area you want hot. In the areas with low AC needs, you only benefit from one side of the transaction, most of the year.
Another consideration, heat pumps are great at providing slow constant heat. The ideal situation is for them to run almost continously. The problem is we don't use energy that way, we use it infrequently and in large amounts. We demand alot of hot water in the morning to take showers and don't use much until the evening or the weekends to do laundry. So if you drain your tank in the morning and replace it with 40F water, you'll have to wait a long time for hot water or have backup resistive heat kick in to make up for the shortfall and reheat the water quickly. This is the same reason you can't use a programable thermostat with heat pump heating, it can't provide heat fast enough to recover and back up heating kicks in.
It still may be beneficial for you, just be aware of all the limitations. Also check for differences in electric prices between winter and summer. In my area, (after 800 kwh) winter rates are 4.5 cent/kwh vs 11 cents/kwh in the summer, this greatly impacts the economics of decisions.