Four broken breathalyzers in Philadelphia affected more than a 1000 DUI cases in that city (article), and suggest that you may be better off declining the breath test. Better the hassle to be detained and taken to do a blood test (I believe that's what happens when you decline) than the major hassle to fight an unfair DUI conviction later.
Update: A quick online search indicates that the consequences of declining may vary depending on where you are in the country.
