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Similar to other metropolitan subway systems, the Boston MBTA is comfortable, clean, and efficient. Regardless of time of day, there was always a train arriving within 5-ish minutes. During rush hour on weekdays between 4pm and 6pm in the afternoon, some of the major stations got more crowded, but we always managed to squeeze on board. Not all lines are the same in terms of quality. At the time of our visit, the Green Line was a much older line. The trains were older, louder, but still got us where we need to go. The Red Line and Orange Line both appeared much newer and were much quieter. Luckily, we did not need to take the Green Line very often and spent most of our time at the Red and Orange Line in Boston.
The single ticket fare when we visited (May 2016) was $2.65 per ride, regardless of distance. A single ticket is issued as a disposable paper ticket (called a CharlieTicket). At various stations (not all stations), you can get a free rechargeable card, called a CharlieCard. The CharlieCard can be loaded with money for single/multiple fare rides, or for various passes. Even though we only stayed 2 nights, we figured getting 1 week pass for $19 was more economical than purchasing two 1 day pass for $15 each or to purchase individual rides. The CharlieCard can also be purchased at certain grocery stores, so if you're visiting Boston for the first time and plan on taking the subway, it's worth doing a bit of research to determine the storefronts or T Station where you can buy a CharlieCard.
An extra tidbit: If you’re wondering why the MBTA tickets and cards all start with Charlie, it turns out it is based off of a play where ‘Charlie’ is forever stuck on the Boston subway station.