I noticed a comment in another thread about so-called 'white' LEDS, and
would like to contribute / ask for comments on experiences with these
beasts.

Experience.  (Kingbright RGB LED - 5mm clear and milky versions tested)

As these are made from three separate LED die in a single epoxy 'LED'
casing, there are some side effects that are not exactly desirable....  The
most obvious is that the LEDs are only white when you are on a specific
vector near-axis.  Moving away from this point the colour shifts towards
the three primary colours and their combinations.  The actual usable white
cone off the observed axis is VERY small.

The blue chip is excellent, and creates a very pleasant blue colour - which
is useful in itself to distinguish display states.

The price becomes reasonable in very large quantities, but the colour
prismatic effect renders it useless as a white led.  Imagine seeing a vivid
RED indicator from an angle - when it is really 'white' (r+g+b) or 'yellow'
(r+g)

Over to you
MC