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Google then assumes the whole query is a phrase. Obviously this won't work for "blue widgets" UK or similar queries.
I quite like 'i'm feeling lucky'. I use it from time to time, and it's a good way of looking at sites that I would ordinarily have passed over while scanning the SERPs.
For example you want to go to Slashdot. You remember the address is not slashdot.com (officially) but don't know the exact url. Hit the lucky button and save yourself a click on the serp.