I think we all really need to re-examine extactly what defines "slasher flick" here.
I used to think a slasher flick was about <insert unidentified deranged killer here> stalking and killing a core group of people one at a time until there is a final showdown between said killer and the last man/woman standing, where said victim would get the upper hand and destroy the killer. (reference Friday the 13th, Halloween, the Scream movies, etc). Presumably, the term "slasher flick" came from the fact that a lot of these movies use some sort of slashing/stabbing weapon (knife, ax, razor, sword) as the killer's tool of choice.
However, in recent times, it has become a lot more difficult to determine exactly what constitutes a "slasher flick", as there are a lot of movies that follow the same basic "slasher formula", but would be hard pressed to call a slasher flick.
For example, take Alien. In the old days, I would define this as horror, maybe sci-fi, maybe a combination. However it does follow the same slasher formula, the alien (deranged killer) kills a group of people one at a time (the crew) until the final showdown between the last man/woman standing (Ripley), where said victim gets the upper hand and destroys the killer.
So would Alien be a slasher flick? Why or why not? It follows the formula. If not, is the difference the fact that the antagonist is alien as opposed to human? Is it the fact that the killer does not use some sort of slashing weapon in the act of killing (other than claws or stingers or some other twisted alien stabbing device)?
So in answer to the question "Are slasher movies SCARY at all?", being the desensitized horror movie fan that I am, for the most part, no. They're formulaic and predictable. Of course, you can never underestimate the power of a well timed spring-loaded cat to make you jump. However, in answer to another question "Are movies that follow the slasher formula scary at all?" I would have to say some, but only because there are some creative people out there still trying to come up with new ways of scaring you (some effective, most not).