I agree completely with Akahitoha's reply. It is spelled with a hyphen: "so-so." Everyone understands it and completely idiomatic. If this is a comfortable expression for you--if it corresponds to something in your native language, for example--you can use it without hesitation. It is often preceded by "just" or "only," "just so-so," "only so-so."
It is a little negative. It means acceptable, passable, OK, good enough, but not great--you would prefer something else if you had a choice, you were hoping for something better. "So, how was the food at that expensive restaurant?" "It was so-so."
It is good standard English, you could use it in writing as well as colloquial speaking, but I think it is a little informal. (Checking the American Heritage Dictionary, 3rd edition, they have no notes about it. They do NOT call it informal or colloquial).
When I say it, I usually make a gesture with my hand--holding it out flat and level and then tipping it a bit from side to side, as if to say "I'm weighting it in a set of scales and the balance is tipping back and forth."