Was previously using a instant heater from Cornell (for more than 4 years, no problem after that I sold my house). Now using the free gas water heater (Rinnai) given by PUB - didn't really like it but slowly getting used to it.
For the cornell (don't know the model), the internal heater was wrapped with a insulating material which the saleman say will safe electricity etc and most importantly the metal heater material will not deteriorate and take more effort to heat up.
Anyway, there are 2 types of water heater, those with 2 temp control knob and those with 1 knob. 1 knob is used to control the heater temperature directly while the second (those with 2 knob) control the temperature by controlling the amount of water that pass thru it. So if your water flow is weak, cannot use those with 2 knobs.