is it safe? as long as you don't overuse it or allow the kitty to inhale the dust yes it is. efficacy of it is (seemingly) so-so, at least based on my use of it with my colony.
i put it in my cats food daily and still have seen tapeworm as well as roundworm in the fur near their anus. however, the 2 of them that i have seen with the worms are very skilled hunters - within a hour of eating their breakfast today one of them had caught a small mouse and the other came dashing up to me and proceeded to drop a large mole out of his mouth - so, at least with these 2 it is an extreme scenario.
to treat them with dewormers would be a never ending battle, not to mention my concern about the toxicity with frequent usage. what i might try to do this year is continue to use the d.e. every day and once or twice give them doses of dontral, at least to the 2 "killers".
remember though, d.e. is not fast acting. so it may be a good idea to see if you can get your vet to give you some dontral or you could also buy some panacur-c which is available w/o a prescription.