I have one right now (used to have three, then one died from an ick battle, and one got really agressive, so I was down to one, he started getting really skinny, and i thought he was lonely, so I bought another one for him, then he died, and i was left with the one, and I have not bought any more yet. I will soon though...very soon).
I know she will be happier once I get some more in there, but as far as surviving goes, she is doing just fine. I do agree with everyone else though, they are much happier in groups. Mine does come out at feeding time though, or when we are doing anything in the tank. I just added my emerald cories to that tank (they have been in QT) and she swims around with them sometimes.