If you're not intoxicated, go ahead and do the brethalyzer. But if you are, it's better not to, unfortunately.
I forgot to mention, you know what ALSO isn't admissable in court (i.e. can't even be MENTIONED)? Previous DUI arrests. If some guy's been picked up 10 times for DUI, the jury won't know it and will only be dealing with the arrest in question as if it was his only one.
I guess the moral of the story is, if you see a drunk driver on the road, run him off and hope he totals his car and kills himself.