Garmin's traffic covers a very limited number of roads compared to Google and Tomtom. There is practically no coverage from B Roads downwards. Navigon uses the same system, which is where I've used it (they are owned by Garmin). Usually what happens is it detects traffic on an A Road or Motorway and diverts you onto roads about which it has no live information, which may or may not be clear.
This is still the case with Garmin's newest models with digital traffic. It's just updated more regularly than the old models.
However Garmin does have an advantage over Tomtom in some circumstances. It had much more extensive details of road closures on secondary A Roads (off the HA's stategic road network). If, for exampe, I wanted to know if the A57 Snake Pass between Sheffield and Manchester is closed due to snow, Garmin / Navigon is usually spot on and very timely. That info is also available in the Inrix traffic app, which is free (Garmin get their closure information from Inrix). With Tomtom such closures are often not detected, or are not shown in a timely fashion.
Google is plain dreadful regarding closures for anything other than Strategic Road Network closures, or closures long-term enogh to make it into map updates.
I currently have Google Maps, Navigon, Tomtom Go and Route 66 Navigate installed and often compare them when I'm aware of nearby traffic incidents.