We have been using smartfox in our isometric 2D social game for a very long time (since the beginning of it in fact). And I wanted to give a review to it for that reason.
First of all, I only know the iphone client of it, because I've only used it in iphone. So this review is valid for iphone client of smartfox.
They made iphone client closed source after some version. Because of that if a problem happens, which is likely to happen, you can not debug it. It is like a black box. And for an unknown reason it's crashing a lot. By a lot, I mean really a lot. Checking the crash statistics of our app I can say 90% of the crashes are smartfox related and number of crashes is in thousands.
Seeing so many crashes and with the fact that it is a blackbox thing, I would unfortunately not recommend using SmartFox for your multiplayer project. There is a forum of it but it doesn't seem to help in getting quick help either.
I don't know if there are good open-source multiplayer game servers but if there are, I would strongly suggest using one of them. A multiplayer game server is just a layer of getting commands and sending events or responses to clients after all. Whichever server you are using, most of the work will be your extension, which is the game logic. What SmartFox provides in addition to that is some default game concepts like rooms, users, banning etc. But they should not be so hard to implement. So I would suggest using a lightweight event mechanism for that. It can be even something like now.js or maple.js if you are into node.js.