Boston weather is hard to predict. If you don't like the weather today, wait until tomorrow!
While it's certainly true that New England weather can change on a dime (a random 65 degree day in the dead of winter, anyone?), generally speaking winters are bitterly cold and snowy and summers are hot and seriously humid. The best time to visit is during the Fall and Spring. Fall brings cooler, crisper air (although there is still the chance for an Indian summer) which makes walking around the city a much more enjoyable experience than in the shirt-sticking-to-your-back temperatures of a typical Boston summer day. Plus, the fall also brings the excitement of baseball playoff season (and perhaps a shot at another World Series title), the beginning of football season and other sporting events such as the Head of the Charles Regatta where hundreds of collegiate rowers take to the waters. Sports are a major part of the Boston experience, so there's no better time to see it for yourself.
The springtime can bring unexpected rain and temperatures ranging from freezing to sweltering, but there's just something about Boston in the Spring that you can't beat. The snow is gone (usually) and the flowers in the Common are starting to bloom. People are happy to shed their heavy coats (as long as some freak cold spell doesn't occur) and you can take in a stroll along the Charles or down Newbury Street in relative comfort.
When it comes to traveling in Boston, it never hurts to pack an umbrella and a jacket because you never know what Mother Nature will throw at you on a day-to-day-basis.