First of all, do not approach this film if you are expecting something similar to the slapstick and sight gags to be found in other John Landis films like The Blues Brothers or Trading Places. This is a faithful recreation of a 1930's style farce, based on a stage play, with snappy dialogue & several cases of mistaken identity. Its stage roots are obvious, in the set design & the performances, but this isn't to say that the actors are on autopilot. Chazz Palminteri & Peter Riegert are terrific in their supporting roles; the scenes between Sylvester Stallone & Vincent Spano are great; Marisa Tomei delivers another good performance and Kirk Douglas' cameo is hilarious. Tim Curry mugs it up a little too much, and the only disappointment is Ornella Muti - She's beautiful but seems to do very little but shout.
I won't go into the plot: that's what Wikipedia is for. It twists and turns a little but the viewer can always see where it's going. (Almost) all the performances are solid, the script keeps everything moving well despite the majority of the plot taking place in just four rooms, and the '30's atmosphere is perfectly recreated. Unfortunately this recreation is both the main strength & the greatest failing. If you're not a fan of '30's comedy or gangster movies of the same era this will leave you cold. It failed to find an audience on its initial release and continues to be criminally overlooked.
Give it a chance... you might actually like it!