Errr, yeah, well ... would have been a bit better if there hadn't been 12 years of forgetting between the course and the trip, I imagine. I did try revising a bit beforehand (revising random bits of vocabulary, and listening to podcasts, mainly), but not everything was sticking, and it was only a fraction of what I used to know.
Still, we got on ok, I did manage to have actual conversations entirely in Chinese (albeit very simple ones, e.g. when buying stuff or going places in a taxi), and occasionally would make sense of enough characters to help us out in places.
I found there were three reactions to me speaking Chinese (especially if I started before they started speaking English at me) :
1. Some people would assume that I was fluent and started speaking full-speed Chinese back at me - fortunately, in context I usually had some idea of what they were saying and could at least ask a question or reply vaguely sensibly (not always, though :-) ). If all else failed I could shrug and say "wo bu dong" (I don't understand) and resort to gestures and simple words ...
2. Some would suddenly go very shy and say almost nothing in any language - had this happen at two restaurants, even though I'd used a little Chinese the waitresses barely said anything and just pointed at things and looked confused at us.
3. And some would just go on and use mostly English at these obviously-foreign people who were obviously just using a few Chinese phrases that they'd learned to be polite. (Well, near enough to the truth, I guess ... )
How much Chinese do you need to know to get around in China ? As always, it depends how far off the tourist trails you're going - around city centres and major tourist attractions, you could probably get by with practically nothing. Further afield, you need to know enough to communicate with shop assistants and taxi drivers and to put off any random touts you encounter ("bu yao", almost literally "Do Not Want", works quite well for this). As for learning the characters, I'd recommend at least learning the basics of writing them (so you can copy characters e.g. in an address, if required) and a bunch of standard ones - I'd only go beyond that if you were serious about continuing to learn the language.