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."

I think English learners use the phrase far too much, and choose it as an automatic response to "How are you?" We native speakers do use "just so-so", but if you're learning English you should use the chance to practise different phrases and responses instead of choosing the same reply every time.

It's used in China a lot, maybe I will start using it when I go back to England! I would use It's ok or It's alright or It's so-so.

I've said it before, I'm a native English speaker. It's generally used when something isn't utterly fantastic or terribly awful, we'll say it's "just so-so." It means it's not particularly leaning towards good or bad, generally.