According to some research, the gold rush is an excellent example of what historians call transnationalism, referring to the non-state interactions which take place across national borders. For example, the gold rush completely transformed Panama, both its society and economy.

When many people think of 49'ers traveling west, they probably think of covered wagons moving across the plains, all that Oregon Trail stuff. However, most Americans heading west traveled by sea, and even before the canal Panama was a transportation hub for those heading between the Atlantic and Pacific oceans. The sudden influx in travel that the gold rush brought to Panama caused numerous conflicts not only between Panamanians and Americans but also internal power struggles.

Americans had racial concerns about Panama as well as resenting the ways in which Panamanians profited off their travel. Local boatmen (who ferried travelers across the isthmus via waterways) and associated travel industries (sex, lodging, food, etc) were making a lot of money off this boom. They resisted the encroachment of American transportation companies and angered Americans by not protecting specifically American interests (this means they didn't keep non-Americans from also traveling). Local elites also were ruffled, as many of these entrepreneurs were everyday people and power relations were shifting.

Backed by Panamanian elites, American companies managed to secure a "corporate zone" and a transcontinental railroad was built by 1855. You might imagine how this ruined the small boatmen and related travel industries, whereas travelers could now cross between the oceans in hours instead of days and had no reason to stay with, eat with, or fuck the locals. As had taken place in Texas and California before they entered the US, Americans often refused to respect local authority. Meanwhile there were labor conflicts between workers and local governments. American transportation companies were making inroads but didn't always conduct their business according to Panamanian law.

The resulting conflicts brought up questions of power and sovereignty, not to mention racial ideology. Some of this was of course centered on Americans. The railroad companies sought to politically marginalize Panamanians who stood in their way and often had the backing of the American government. Though Panama was not officially colonized, at least one author characterizes their relationship as resembling that between a colony and a metropole. The US was very much concerned with its commercial interests and securing the flow of people and goods as it saw fit, even if it didn't specifically seek to control Panamanian territory outright. This situation can be grouped with things like opening ports in China and Perry's "gunboat diplomacy" in Japan, as well as broader US policy concerning Latin America. Though the US didn't seek overt colonial possessions until 1898, examples such as this show how foolish it is to ignore how imperial the US behaved throughout the 19th century, though its empire might take multiple forms.

Internally, the gold rush served as a catalyst for the transformation of Panamanian society. The power struggles and political debates over citizenship and internal racial divisions, such as the treatment of indigenous populations and racial divides (Panama had perceptions of whiteness and blackness even without US influence). Violence erupted repeatedly as a result of these conflicts, with blacks resisting the power both of Americans and local elites. Ultimately the result was expanded notions of citizenship, such as the extension of universal manhood suffrage and the abolition of slavery, as well as expanded rights for women.

Source: Aims McGuinness, Path of Empire: Panama and the California Gold Rush

I'm sure someone will be along soon with more info but, just to keep things in perspective, at the time, most of the actual circulating money in the U.S. was Mexican silver (which kept coming for centuries, as opposed to Cali's flash-in-the-pan) and the States' prime export was cotton and other raw materials farmed or gathered by black slaves.

There was some industry (much of it owned by British, Dutch, &c. capital) but it wasn't worth as much or as important as the Southern exports. I'd imagine the general attitude in Europe depended on class: the lower class considered it a haven and the upper class and ecclesiastics considered it a somewhat barbaric backwater. Fistfights in a spit-tobacco-covered capitol didn't do much to improve our image.

edit: For what it's worth, I'm sure Mexico was pissed.