My experience with the block heater is that it only helps the starter turn the engine a little faster but does not really help the glow system or anything else.
At minus 2 f my 240D starts about the same with or without the heater (gotta kind of wish her to life) but with the heater plugged in the starter definately turns the motor faster.
Just my experience...

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique