"Forum" is one film adaptation of a Broadway musical that is a half-glass of water. On the half-full side we get to see Zero Mostel and Jack Gilford reprising their Broadway roles, and they're joined by a fine supporting cast including Phil Silvers as Lycus (ironically, he would star as Pseudolus in the 1972 Broadway revival) and Buster Keaton in his last film. But even though the film delivers the laughs and opens itself up well from its stage origins (let's face it. That climactic chase in the Broadway version is wonderful but it just wouldn't work on film at all). But what I can't forgive is how nearly three-quarters of Sondheim's outstanding score (his best I think) has been jettisoned to give us a too short running time of barely an hour and a half. And on top of that, why do *both* of Milos Gloriosus's numbers get kept, while Mostel's two best numbers from Broadway, "Pretty Little Picture" and "Free" (which is supposed to be the heart of the show) are gone (along with Gilford's "I'm Calm")? That, I do not understand at all. It's gotten to a point where I purposefully stop the film after Psuedolus falls out of the tree so I can then play Mostel's version of "Free" from the Broadway cast CD (which is where it would have belonged in the film). And it's too bad this wasn't a roadshow movie with an intermission because Mostel's funny Act One closer would have worked great there as well.
All in all it's worth having, but be prepared for your mood to shift from seeing the film as a half-full glass or a half-empty one.