They are very simple to use and improve playability vastly when put in the right places.
What they do: If you try to put a portal in an area that would mean it crosses a portalbumper it shoves the portal to the nearest space that doesn't collide with a portalbumber or non-portalable texture
Handy for: Bits where portalable and non-portalable textures meet.
If you don't use them: The player can fire a portal on a section of concrete that is close to a metal texture and have thier portal rejected. The player feels cheated and is made unhappy, like this -
If you do use them: The player can fire a portal at exactly the same place and the portal is accepted (Note - the portal doesn't overlap the metal texture). The player continues as though nothing special has happened, completes your level and feels that much happier, like this - 
How to use them: Make a thin brush and texture it with an invisible or nodraw texture and place it along the metal side of the join, you don't need to bother at corners. Make sure it is right against the wall and hit CTRL + T to tie it to an entity and make it a 'func_portal_bumper'
(I prefer invisible because you can see through it, nodraw just hurts my eyes)
Acceptable excuses for not using them: You're retarded.
From this point on, I expect to see no more maps without them, because they are so simple to put in place and are so very helpful
See picture if any of this is confusing:

