Saw this for the first time just now. Pretty fantastic, I especially liked how the film allows you to get as annoyed with Fonda as the other jurors are, before he starts making his case. All the actors nail it, even if half the characters are little more than types, but they are types convincingly brought to life, and Lumet perfectly sustains the energy for 96 minutes. I never even thought of checking my watch.
I do have some nits to pick with it - all the attention being called to the "hottest day of the year" and the non-working fan is the kind of thing that tends to annoy me ("it's because they're gonna have a heated, emotional argument! Get it? Get it?!!"), and when the old guy started his racist monologue, everyone demonstrably turning away from him - capped off by "Now sit down and don't say another word" - felt like a reaction that was performed for the audience more than something that would've happened in real life. (I can easily believe they'd get disgusted with him, but not in such a stagey way). Still, the fact that those things stood out to me is, in a way, a further testament to how accomplished the rest of the film is.