"If you have to rely on London Underground to get you to the airport
everyday, this service will revolutionise the way you travel." Some
revolution!

I never understood the real point of services from paddington. Unless
you
live in or near the paddington area or routes leading into it then
you've still got to crawl around on the circle line to get there and
wait
for the service to leave.

Well, you could always use the Bakerloo...

By the time you've done that you could
probably
be halfway to heathrow on the piccadilly line and have paid less than
half the price.

HEx is not supposed to be a substitute for the Piccadilly line. It is
not competing with LU for traffic, hence the extortionate fares. It is
supposed to compete with taxis; HEx to Paddington then taxi onward to
destination in central London being both faster and cheaper than
taking a taxi for the entire journey.

Surely if they want a really useful airport service they
should extend the 3rd rail system from south of the river to heathrow
too?

You seem to be assuming that providing a really useful service is one
of the aims of the privatised rail industry. See also the threads on
the London-Brighton bike ride, etc.