This system is an above-average way to get a quick look at Phoenix for basically nothing (day passes are only $4 from ticket vending machines). If you're stuck on a long layover at Sky Harbor, take the Air Train to the light rail connection at 44th and Washington and head west, towards 19th/Montebello.
Heading into Downtown, you'll pass by a few sites including the homes of the Diamondbacks (Chase Field) and Suns (US Airways Arena), see some everyday Phoenician businesses and avoid having to deal with parking. A few stops offer food options as well, within a short walk in most cases. End-to-end, the line is about an hour's trip.
There's art, some shade and even a few placards about the area's history at stations too (interested in reading about suburban development in the Phoenix area? Central/Camelback station, towards Sycamore/Main). That the system seems to run on time, is clean and offers handy bike transport is great.
Note:
* Valley Metro's system map is available here (http://www.valleymetro.org/images/uploads/sysmap_011414.pdf). The heavy gold stripe in the middle? The limitations of the system.
* Want to get some ideas of places to visit via bus + rail? Valley Metro offers a Destination Guide (http://www.valleymetro.org/images/uploads/destinations_newsletter/2014Destination_Guide_for_web.pdf).