Lots of traffic most of the time, but wide lanes make it fairly easy to pass slower users. Many parks and nature observation areas to enjoy along the main trail. Long, gentle grades; beautiful scenery; great people-watching; ideal for commuters.
Flat and shady, this trail is a nice ride! There are lots of pedestrians and other bikers, but most people are friendly, courteous and will share the path with no problem. There is a bike shop that has a door open to the path if you need to make a repair. There are lots of restaurants and shops along the way to get a bite to eat or a drink. There is a very short (literally crossing one block plus crossing a busy street with a light) part of the path that is not restricted - other than that, it is restricted to recreational use.
As one of the most heavily-used trails in the country, it gets pretty crowded. You get a diverse mix of commuters, road bikers, rollerbladers, pedestrians, joggers, one or two random guys on an elliptical-bike, and lots of parents with strollers (one of the segments on Strava is named "Stroller Slalom" - HAH!). It usually gets better after you pass Trader Joes in Arlingon. My favorite parts are by the Meadows in Arlington/Lexington and past Lexington, when the trail gets a little 'wilder'. Several sections of the trail have been recently repaved and the lane markings on the whole trail repainted. Bonus points: you can bike all the way to Davis Square. Or, if you have the right bike, you can continue on the unpaved Reformatory Branch Trail in Bedford. You can hop off your bike and do some hiking in the Great Meadow in Arlington or the Parker Meadow Conservation Area in Lexington. In Bedford there's Hartwell Town Forest with some trails. Check OpenStreetMap for more info. Oh, an the trail gets plowed in the winter - how great is that!