In the book Twenty-One Balloons by William Pene du Bois, what does Professor Sherman like about the escape balloon?

Professor Sherman, in The Twenty-One Balloons (Dubois), retires and realizes his lifetime dream of drifting away in a balloon. Circumstances conspire to land him in Krakatoa, where he encounters twenty families who have happily settled there, living on the wealth of the diamonds they have found on the island. (Krakatoa is a real island, nearly decimated by a volcano in 1883, but there is no reason to believe there were diamonds mines on the island.)  When the volcano on Krakatoa erupts, the families and Professor Sherman escape on an ingenious escape vessel, essentially a large raft that is lifted and moved by means of enormous hydrogen-filled balloons. 

As a balloonist, Professor Sherman appreciates any means of transportation by balloon, and as a professor of math for many years, he has an appreciation for calculations, too.  This escape raft is particularly well-planned and attractive.  The sizes of the raft and balloons to transport eighty people, twenty families of four, and now Professor Sherman, have been carefully calculated, and the raft and balloons have decorative elements that will make it more palatable as it goes over the land somewhere, so that they appear to be civilized people, rather than some sort of marauding raiders. Each family has a "group" parachute, since the raft is far too large to land.  A means of quickly inflating the balloons has been worked out, and all that is left to do is have a personal parachute made for Professor Sherman. As it turns out, there is no time to get that parachute made because the volcano erupts as never before. 

In spite of the fact that this balloon raft has not been able to be tested, it does work surprisingly well, getting the families and Professor Sherman safely away.  The decision is made that nineteen of the families will parachute off at the first available opportunity, leaving Professor Sherman and one family to steer the raft until they are near water, where Professor Sherman can try to land the raft and the remaining family can jump off right before the water.  The nineteen families parachute down in India, and the F family and Professor Sherman keep going.  The F family leave him to parachute off in Belgium, and Professor Sherman manages to crash the raft into the Atlantic Ocean, where he is rescued.

So, Professor Sherman and eighty other people have ample reason to like the escape vessel. The careful planning has saved all of their lives.