The message
from Tim S contains these words:

I've been happy with the Garmin Nuvi, although I've been in a car with a
TomTom and my Nuvi side by side[1] and the TomTom possibly gives slightly
better verbal directions.

I have a Garmin Nuvi which really ****es me off. I bought it as it was
the only in-car unit around at the time that would take grid references.
As I go hill walking my destinations often don't have post codes.

There are a few junctions where there is an obvious error (which
presumably is a problem with all units) and literally thousands of
junctions where the instruction does not match the road priority but my
principal beef is with the treatment of single track roads. Whoever
coded the average speed for these roads obviously doesn't have a clue
about the difference between single track and single carriageway roads.
Generally I find the ETA reliable but with single track roads the ETA
becomes hopelessly optimistic with the expected average somewhere
between 150 and 200% of that achievable with any degree of safety.

--
Roger Chapman