Pack n play for crib

So I've been thinking of just using a pack n play for a crib. I told my best friend about it the other day and she looked taken aback by the idea like I was doing something highly dangerous. Instead of saying anything though she just walked off (we were in a store) she doesn't usually have reactions like this so now I'm worried. So here's my question: do you think a pack n play is safe to use for a crib?