Boilers

A quick and unscientific survey reveals that most still have the original 20-year-old heating boiler, typically a Glow-worm Fuelsaver 55F, and most of those have been warned by their local friendly plumber that it can't last much longer.  Or can it?  British Gas service engineers express few concerns about the boilers and, unless it's starting to make a noise, you should have little reason for concern.  Those that are being serviced annually are showing very few problems.  Even ones that are now heating an extension are doing fine.

What are your experiences of those ageing boilers?  Are we being pushed towards replacement when there's actually no need?  And if you have replaced, what have been your experiences?  What boiler have you used and how much have you ended up paying?