The spell Planeshift works a bit different in TDDC than in other settings.
You cast the spell and before you appears a portal that is only visible to you and your traveling group (but most likely true seeing still let's you see the portal of others.) When you go through the portal you end up in one of the two planes you choose beforehand, but not in a place you decide or random, but in the corresponding place where you opened the portal.
That means if you would Planeshift in front of the castle in the capital castle you would wind up in the corresponding castle in the Shadowfel (or Feywild.)
The portal stays open for Caster level/days.
If you don't reach your portal before it closes you will suffer from plane lag (what mechanical propblems that brings is not currently known but it has to be unpleasant.) When you go from a natural portal you won't suffer from Plane lag.
If you use Planeshift you can't use the spell slot from which you cast the spell again till the portal is closed either by you closing the portal or going through it again to automatically close it too.
There are 3 planes and because you are already in one you can only choose 2... Well I think that this is the case it would seem rather pointless to open a portal to your plane because you would just end up at the same place... I think. Or that is the secret way to access to two normally unreachable planes, who knows.