BOS is great for spotting, although, you won't be as impressed as you were at JFK, obviously.
For spotting, head in the direction of the arrivals for Terminal B (American etc.) Then, you will see parking there (bear left_. Go to that parking lot and go all the way up to the top level. Once you're on the roof, keep going straight until you can't go anyfurther and turn left. That corner of the garage is the best area. You can also just cruise along the rooftop. You'll be able to see the AA and UA terminals easily as well as the USAirways and USAirways Shuttle (and CO and DL Shuttle) terminal VERY well.
Recommendations: VS and BA arrive between 12:15 and 1:30p. A lot of the other Int'l flights arrive between 3:00-6:00p. You will see a Swissair 743 taxi to the DL terminal, a VS 742 taxi to the area between the UA and AA terminal and a Sabena A330 or A340 taxi to the DL terminal as well. An AF 763 arrives around 6-6:30p and the Alitalia 763 departs around the same time. This is all from my experience, but things always change. You never know what you will see at BOS.