Two questions on the invisibility cloak

[Ersatz Harry]

No: HPFGUIDX 58277

These may have come up before, but...

First question:  If the cloak makes you invisible, why is the cloak
itself apparently so easy to see?  I can think of three or four
instances where people see it.  First is when Harry receives it; both
he and Ron can see it.  Second is when Hermione retrieves it for Harry
from the hidden passage to Honeydukes.  Third is when Snape finds it
near the Whomping Willow (or in the passage to the Shrieking Shack),
and this occurred after sunset, not a time of day with much light. 
Fourth is when Lupin pockets it after all the craziness in the
Shrieking Shack.  There may well be others.

Second question:  When Ron first sees the cloak, he notes that it is
both "really rare" and "really valuable".  Offhand, the only other
person I can think of who has one is young Barty Crouch.  Does the
rareness of the cloak have some larger significance?

Ersatz Harry