Hugo,
Indeed it's a bit frustrating for me to view the 3D bars (that look
like huge skyscrapers) but I cannot click on them and get a info
window reliably. Due to their 3D nature, it's also difficult to get a
sense of relative size among cities: I had to tilt the globe so that
tall bars line up and become 2D bars so that I can see which bar is
bigger.
My two cents is that it'd be visually easier if the bars are just 2D
icon, the size of which indicates how big a metro/city is in terms of
population. The downside is that big cities with big 2D bars might
have more surrounding areas covered and blocked from view. Maybe you
could make the 2D bars/icons small enough and transparent to start
with, and upon hovering, they turn into something more informative.
Hope that helps.
-ss