Just finished up a great cruise to the Dominican with Fathom. It was a very different ship, a very different atmosphere, and a very different demographic on board. And we liked it a lot. What originally got our attention was the seemingly great value, but we were intrigued by the "social impact" idea, as well.
The ship itself is smaller and boxier than any of the newer mega ships, but the interior is beautiful and will remind you of what luxury cruising was like 20 years ago. More importantly there is a great feel to the ship, led by a very very nice crew and friendly officers. We never had anything but great service.
As for the volunteering, this is not 3rd world mission work. We did three half days, and referred to it as "volunteering lite." Air-Conditioned buses with a good support staff made these safe, fun, and rewarding days. We were then self-indulgent at the pool or beach in the afternoon....and shared a bottle of wine at night with our fellow travelers. Like I said, this is not Haiti or Bangledesh.