Both
MHT and
BTV have one.
BTV's is their old control tower, overlooking the
UA/US/Common Use (I've seen
AA,
DL,
NW, and
CO all there before) gates (can't really see the
B6 gate).
MHT is right at the corner of the old A and B concourses, right over the Southwest, United, Delta, and US Airways gates. You can't really see the old C concourse from this location.
Both are open to the public, with
BTV being open at specific hours (I think its 9-6), while
MHT is open 24 hours a day (but the airport is effectively closed down between 0030 when the last
DL flight arrives, and 0530, when it turns back for
CVG). I've used the deck at
MHT as early as 0630 before to go spotting, and I will be spotting from it at 0530-0600 next month before flying off to
ATL.
Jeff