I have a tomtom with live traffic update - i assume itll be roughly the same as the garmap ones
I can say it exceeded my expectations massively - i thought it would be rubbish to be honest.
It sometimes says the traffic problems start later than they actually do (and end earlier than the actually do) but i guess with dynamic traffic flow.
But having said that - it actually works and for me has a very calming effect knowing more or less how long the seemingly never ending congestion is going to last.