Hi
I gamemaster CoC regulary.
PositivesIt is tons of fun !!!
The % system is very foolproof, even people how have never rpged before get it right away.
The rules are simple and do not get in the way
It does mundane detective stories very well
The rules have hardly changed in seven editions, you can easily use older adventures for 7ed.
NegativesCharacter creation is time consuming
Your characters are expected to go insane and/or die. Running from the monster is usually a good idea ( think the alien movies).
Make shure your players know this beforehad....not everyone is okay with this type of game.
the cthulhu mythos is very well known...often it is better to invent your own monsters...otherwise the players will not be afraid and instead go...oh its a ghoul, gotcha.
Rule 1 with CoC...never describe the monster compleatly, never mention its stats !! Instead of saying: "you see a xenomorph at the end of a hall " say something like. " you hear an inhuman hiss...as you look up you see a huge shadow moving in the darkness...
..your players will always asume the worst and freak themselves out ...it#s great

Cheers Seras