I had heard about sage a while ago. I had figured that I'd give it a try if I needed it over Mathematica and friends. That need arose a short while ago. And I tried using sage for some hybrid symbolic/numerical computation. So, how was it? Read on.
a) The online sage notebook experience is simply awesome. A very good idea. May be, somebody should do an Android/iOS app for this. It could be quite useful.
b) The symbolic manipulation capabilities are decent. Mathematica's are demonstrably better at this time, but the difference I saw was that the expressions were simplified better in Mathematica. This is hardly a exhaustive review, but it wasn't a deal breaker in any way.
c) I tried evaluating the error function for a complex argument. Sage went poof. The expression was complicated, so I didn't wan't to get my hands dirty with it. But the milestone is set to be the next version, so it's not that bad.
Bottom line, Sage is a very good step in a very good direction. But it will be a while before it crosses the reliability threshold. My guess is 3 years with decent amount of funding.