I've always used normal ones, once had a hot one in
my savage for a while and did not feel any difference,
later I was advised by a mechanic friend to remove it
because of the constant long roadtrips I took on my bike,
said that under the wrong circumstances I could even melt
a hole on the piston, how cool is that!

...just get a "cold" iridium plug, they seem to not only last
longer but help start the engine even with a weak battery.  

---

Here's some wiki info on heat range:
(Yup, now you have to read, sorry  )

The operating temperature of a spark plug is the actual physical
temperature at the tip of the spark plug within the running engine.

This is determined by a number of factors, but primarily the actual
temperature within the combustion chamber. There is no direct
relationship between the actual operating temperature of the spark
plug and spark voltage. However, the level of torque currently being
produced by the engine will strongly influence spark plug operating
temperature because the maximum temperature and pressure occurs
when the engine is operating near peak torque output (torque and
RPM directly determine the power output).

The temperature of the insulator responds to the thermal conditions it
is exposed to in the combustion chamber but not vice versa.

If the tip of the spark plug is too hot it can cause pre-ignition
leading to detonation/knocking and damage may occur.

If it is too cold, electrically conductive deposits may form on
the insulator causing a loss of spark energy or the actual shorting-out
of the spark current.

A spark plug is said to be "hot" if it is a better heat insulator,
keeping more heat in the tip of the spark plug.

A spark plug is said to be "cold" if it can conduct more heat
out of the spark plug tip and lower the tip's temperature.

Whether a spark plug is "hot" or "cold" is known as the heat range
of the spark plug. The heat range of a spark plug is typically
specified as a number, with some manufacturers using ascending
numbers for hotter plugs and others doing the opposite,
using ascending numbers for colder plugs.

The heat range of a spark plug (i.e. in scientific terms its thermal
conductivity characteristics) is affected by the construction of the
spark plug: the types of materials used, the length of insulator and
the surface area of the plug exposed within the combustion chamber.
For normal use, the selection of a spark plug heat range is a balance
between keeping the tip hot enough at idle to prevent fouling and cold
enough at maximum power to prevent pre-ignition leading to engine
knocking.

By examining "hotter" and "cooler" spark plugs of the same
manufacturer side by side, the principle involved can be very clearly
seen; the cooler plugs have more substantial ceramic insulators filling
the gap between the center electrode and the shell, effectively
carrying off the heat, while the hotter plugs have less ceramic
material, so that the tip is more isolated from the body of the plug and
retains heat better.

Heat from the combustion chamber escapes through the exhaust
gases, the side walls of the cylinder and the spark plug itself. The heat
range of a spark plug has only a minute effect on combustion
chamber and overall engine temperature. A cold plug will not
materially cool down an engine's running temperature. (Too hot of a
plug may, however, indirectly lead to a runaway pre-ignition condition
that can increase engine temperature.) Rather, the main effect of a
"hot" or "cold" plug is to affect the temperature of the tip of the spark
plug.

It was common before the modern era of computerized fuel injection
to specify at least a couple of different heat ranges for plugs for an
automobile engine.
a hotter plug for cars which were mostly driven
slowly around the city,

and

a colder plug for sustained high speed highway use.

This practice has, however, largely become obsolete now that cars'
fuel/air mixtures and cylinder temperatures are maintained within a
narrow range, for purposes of limiting emissions.

Racing engines (and motorcycles), however, still benefit from
picking a proper plug heat range. Very old racing engines will
sometimes have two sets of plugs, one just for starting and another
to be installed once the engine is warmed up, for actually driving the car.

Some good additional info here:
http://www.dansmc.com/sparkplugs1.htm

L8r guys