Lets take the dot-com boom as an example. There were people saying that dot-com companies were overvalued in 1997, when the Asian currency crisis hit. Those people were, in retrospect, probably right. However, the next two years made them look like complete fools, as the bubble maintained exponential growth for another two years before completely collapsing.

Same thing with China. Yeah, the bubble might have another couple of years left. If you get in now and get out when the crash hits you'll make a modest profit. But, if you mistime the crash, you could stand to take huge losses, as the value of your currency investments falls far below the price you paid for them.