Robert11 wrote:

b.**are*they*generally*considered*to*be*cheaper*to *operate*than*the*stand
alone tank types (I live in the Northeast) ?

Fuel cost is less. Convenience may vary with use.

c.**are*they*very*popular*these*days*?

Depends on what you define as popular.

d.**pros*and*cons*?

Tankless units generally cost more than tank units.
Fuel cost is generally lower than tank units.
Depending on capacity, a tankless may or may not "run out of hot water".
If one planned to use a solar hot water heater, or other conservation
measures, it would be reasonable to spend more money for a tankless.
If one was not concerned with conservation, the bottom line would recommend
a standard tank water heater.

Note: if you plan to attach a solar hot water heater, make sure there is an
anti-scald feature, that detects inlet water temperature. Such models will
mix cold water with the scalding water to prevent injury.