The laws and practicalities regarding DUI (or "DWI") are important to understand. Here's a brief basic primer:
1) You can be arrested for drunk driving whether you blow in the tube or not. All the officer needs is PROBABLE CAUSE to believe you are driving while intoxicated. A breath reading is evidence -- strong evidence -- against you, but it is NOT necessary for a conviction. So, if a guy is driving erratically, slurring, staggering, and stinking of beer, a cop can arrest him and a jury can convict him. As a prosecutor, I tried many refusal cases and won.
2) Anything you say can be used against you. Most DWI defendants ADMIT to drinking alcohol before driving. This is corroborating evidence of the officer's observations of an intoxicated condition. While most people know not to respond to police interrogation without a lawyer, it's harder to remember that after a couple of six packs.
3) REFUSING to take a breath test has its own serious consequences. To discourage refusals, many states make the fines and suspension/revocation terms for refusing the test even worse than those for taking the test. Further, some prosecutors will deny a reduced plea offer to defendants who refuse, forcing either a trial or a plea to the charge.
4) In cases where a person has only consumed two or three drinks, taking the test may EXONERATE the person from a DWI charge in some jurisdictions, and result only in a ticket for a lesser offense. Each beer, shot or mixed drink is typically the equivalent of around a .02%, and alcohol is dissipated at the rate of .02% per hour (these are rough but generally accepted principles). In many states, the level for DWI is .08%.
In states where a non-criminal plea bargain is available, most experienced criminal lawyers generally recommend TAKING the breath test UNLESS the situation includes an aggravating factor that typically preempts a plea bargain, such as an accident or a prior alcohol-related offense.
Now, saying all that, may I make a public service announcement? Unlike gear, booze has been involved in thousands of fatalities, particularly on the roads. Having been recently hit head-on and injured by a drunk driver, I say please, DON'T DRINK AND DRIVE. If you're gonna drink, designate a driver. Seriously.