If you're in Antigua and can't get to Montserrat proper -- and it's a very real possibility due to the horrid air service provided exclusively, if sporadically, by Winair -- you can still get an up-close look at the Soufriere Hills volcano and Plymouth by taking a helicopter tour.
Caribbean Helicopters offers a Montserrat tour for $240 per person that includes views of Montserrat's east coast, the Tar River Valley, the Soufriere Hills volcanic dome, the windmills of St. George's Hill, the abandoned airport, and Plymouth. The tour lasts about 45 minutes.