Well that's sort of a situational thing. If it's in plain sight then usaully (at least in nintendo's case) then there is usaully some sort of challenge to get to it. For example:

This star coin is very clear that it's there, but it is challenging to get it.
In conclusion, star coins in plain sight can be quite a good thing, but I think a mix of star coins in plain sight and ones not in plain sight is best.
There's a whole topic just based on level design here:
viewtopic.php?t=19096