Missy
There are a number of considerations given prior to using flex power. Wake turbulance from preceeding aircraft(not used if prior ac is a heavy or 757) weather, runway conditions and length, terrain and obstacles, special departure proceedures, aircraft status, such as an anti skid channel is deferred, certain flap settings peclude its use, tailwind componants. There are a few more but I don't have my SOP in front of me.
That said, I estimate around 3/4 of the time we use flex. As mentioned by Mike, they are good for the engines. I have never actually seen it, but hear often of a study in the US I believe that has shown there to never have been an engine failure during a flex departure.