it depends on what your skin type is how sensitive your skin is and how much sun exposure you are going to have. i love foundations that have sunscreens built into them. i think it's great to add sunscreen into any product you can to get that extra added protection. however i think it is important to use some protection underneath them. i don't think they give enough protection because to have that true spf 15 you would have to use enough product and typically that would be too thick a foundation to really give you enough protection. so i would use a moisturizer with sunscreen underneath and then a foundation with sunscreen in it right over it.
if i use a foundation with sunscreen do i really need a separate sunscreen too?