I know a lawyer who was convicted on one of those. The detectives had the mic + transmitter taped to his groin so it would get through a pat down. Then the undercover just spoke both sides of the conversation (the defendant's side was whispered and muffled). Someone testified it sounded like the defendant. Conviction was overturned on appeal on a simpler issue unrelated to the evidence.
In fact, I remember a drug dealer I was helping with his defense. Hidden mic on undercover was taped under his armpit. He arrived to defendant's hotel room for a deal and defendant made him undress. The recording is hilarious because defendant is like "You fucking snitch, what's that under your armpit?" and the undercover says "It's my .. er .. MP3 player?" LOL