European investors are pretty bummed. Over the past year or so they've seen the value of any dollar-denominated fall substantially. The problem with investing in foreign assets is that you experience not just the underlying risk of the investment, you also experience exchange rate risk.
As for the longer run, negative effects on the US economy of a falling dollar would be more likely to be due to increased uncertainty about the currency and expectations of further falls, rather than the fact that it's fallen as far as it has. Market expectations are really key.
Countries which have pegged their currency to the dollar are also obviously affected.
...Steve Kyle brings up an additional issue, the possible replacement (full or partial) of the dollar as the world's reserve currency. If this happened rather slowly it wouldn't be such a big deal for the economy, though it may negatively affect our geopolitical dominance somewhat more. I seem to remember reading estimates somewhere that if this happened the Euro-zone would get a couple extra GDP points, so presumably roughly the inverse would happened to the US. Not that big of a deal. On the other hand, if this process were to happen rather quickly - and panic about the dollar set in (if the musings of the Yoshidas of the world were feared to be policy) - that could be, as they say, Bad.