I would like to have one. It has a very unique character, a special flavor and are very interesting to look at.
They have also this less traditional though very classical elegance that define many Patek watches.
Now, when dealing with this type of shape, it is really a question of taste. Much more than for a round watch.
The Pagoda, the 5100 and the 5200 are very interesting in that field and if someone tends to go that route, I think they are excellent choices.
I must say that I don't really know what their prices are and won't compare the watches on that level.
I would say that the main criteria would be how I like them on an aesthetical standpoint. Thus the movement would be less important than for a round watch for me.
The Pagoda has a little old shool vintage touch, the 5200 is more modern and the 5100 stands between both as it has a nice Manta shape but with several modern elements as the movement, the dial layout etc...
The 5500 is 40mm long (150 pieces in P) while the 5100 is 46mm (300 pieces in P). That's a major difference.
I don't know about the collectibility etc.... But I would tend to choose the 5100 or the 5200 as I would take into account the movements. But that's just me and the 5500 would I think look better on the wrist because of its size, especially if I wanted to wear it for dressy events.
I think he should have a look at the 5100 before deciding.
That's a great project
Best, Mark