When I first saw this movie as a teenager I loved the courtroom drama and attendant plot twists. But Wilder also has a rare genius for creating fully-fledged comic characters with a few deft, witty lines, and that's what delights me now. Charles Laughton is hilarious as the acerbic barrister Sir Wilfrid Robarts, battling with a fussy nurse, a cranky elderly witness, and a sarcastic judge, and the film has several laugh-out-loud moments. I still think the plot is entertaining, but it does have some mighty creaky sections. The screenplay is based on a short story and play by Agatha Christie, and Christie's plot twists are perhaps more notable for their ingenuity than their plausibility. Incidentally, the short story has a different and to me preferable ending.
It's true that Tyrone Powers is pretty hard to watch, but that's not all his fault -- he's given some terrible lines to work with, and after the past decade of seeing stony-faced defendents on Court TV, it seems crazy when Powers starts emoting all over the courtroom. As for Dietrich, her performance varies wildly. At times such as the flashback scene to Germany, she's very naturalistic -- interesting, even fascinating to watch. At other times, you can almost hear Wilder yelling through a megaphone, "Now let's see some ACTING", and Dietrich, like the British soldiers in the trenches of Ypres, goes bravely over the top.
Overall, however, "Witness for the Prosecution" is great. Itt belongs to the set of films that I will always stop and watch while flipping through the cable channels late at night.