I've owned a Magellan Roadmate 1470 (stolen) the Garmin 755T (gave to my dad to go back to Magellan) tested a friends new Tom Tom and now own the Magellan Maestro 4700 along with an iPhone.

The subscription free GPS systems use an FM receiver to get traffic incidents (same type service that feeds normal radio station updates) - my experience is they are almost always delayed and they can't report the impact an incident is actually having on traffic (i.e. they know an accident was reported but don't know if it's blocking traffic or not).

Google maps (free on most PDA's including iPhone) does a better job by indicating traffic speeds via color coding the road (green, yellow, red) but it's not always the best GPS - interrupted for phone calls, no text to speech and dependent upon cell signal. It is far superior to GPS systems in the "points of interest" category because you get a full Google search instead of limited and out of date information that the GPS systems have pre-programmed.

Magellan has added a predictive traffic routing feature to their new Maestro lines. It take historically bad rush hour traffic areas into consideration when determining your route (based on the time of day that you are traveling). The newer Magellan's have surpassed Garmin in the area of how quickly you can get the GPS to detour your route or avoid a maneuver all together. It's not bug free yet but the Maestro 4700 is my current favorite - MSRP $299 (currently $249 on Buy.com).

I like GPSmagazine.com for reviews - the user forums are connected directly to the review. Great for getting the latest info on impacts software updates have had on units and other real world reports.