Most of the negative comments posted below seem to be from people who either just don't like musicals or who are unaware that all the "cliches" in this movie were essentially invented by "42nd Street." It's sort of like complaining that Shakespeare is full of quotations. This movie is absolutely brilliant, which is why it's been imitated endlessly for the last seven decades.
Sure, Keeler's not the end-all of tap dancing, but she fits the bill as an ingénue and is generally amiable and perky. The plot is predictable, but only because we've seen it duplicated so often. If you hadn't seen the same sort of thing a million times, you'd notice that it's tightly assembled and even somewhat suspenseful. The show is full of first-rate comic asides, even if some of the material is dated by obsolete slang and contemporary pop culture references.
And do people still take the trouble to complain that Busby Berkeley's dance numbers couldn't have been seen properly by the audience in the theater? That's like complaining that an ape couldn't really grow to be as large as King Kong. The whole point is that it's a movie, and Berkeley is able to do things that can't happen in the real world. Hence the transformation of background settings while the camera is close up on an actress's face. There isn't even such a thing as a close-up in a stage production. Carping that a '30's musical isn't realistic enough is like complaining that Venus couldn't actually have been born out of a clamshell.
In any case, this is one of the great '30s musicals... and one of the great Hollywood movies of all time. If you don't like the genre, then so be it. It always amazes me that so many film fans strongly prefer "Singin' in the Rain" to such predecessors as "42nd Street," "Dames," "Top Hat," "Swing Time," etc., when "Singin' in the Rain" is simply an homage to the '30s musical and generates quite little fresh material of its own. Mind you, it's a brilliantly executed homage, and it arguably benefits from its overt tongue-in-cheek attitude, but I can't help thinking many are simply swayed by the fact that it's in color (really good Technicolor) and has clearer sound quality than its '30s predecessors. Either way, you need to see and appreciate the original movie musicals before you can really understand what "Singin' in the Rain" was about... just as you should see some Hong Kong action flicks and blacksploitation films to get what's going on in "Pulp Fiction."
But I digress. See "42nd Street," and try to keep an open mind. Just because it's old is not a reason to assume that the people who made it didn't know their business extremely well.