I recently attended a Chicago ASPE Chapter meeting that had a presentation on heat pump water heaters. The presentation was excellent. The information and the way it was presented was extremely beneficial to the plumbing and mechanical engineering community.
Basically, the program pointed out that heat pump water heaters will be a part of your future. There is no denying this fact. Furthermore, heat pump water heaters and heat pump hydronic systems will become more of a standard product for engineers to consider in the very near future. While products are currently available, as the program pointed out, the major switch will be to low-GWP refrigerants.