Thoughts on on-demand / instant electric hot water heaters?

[Ken P.]

I have a number of 1 BR 1 BA apartments with aging hot water heaters.  I replaced the last one that failed with standard 40 gal electric hot water heaters, but am now thinking of switching to 18kW on-demand / instant electric heaters, at least for the lower apartments.  

Currently it costs my tenants $45 to $50/mo just to keep the water warm in the large storage tank type traditional heaters, which is a significant burden for $550/mo apartment renters.  The units are largely occupied by single people, and I would target any conversions at those units with just one tenant, particularly on the ground floor where the pipe runs are very short.  Power supply is not a problem, as the electric service was upgraded and high amperage 220V panels are just next to the installation area.

Does anyone have experience with tankless electric HW heaters vs. traditional 40-50 gallon HW heaters?  Problems?  Utility savings?  Brands to avoid, and positive recommendations?