Compared to other Harry Palmer films, Midnight in Saint Petersburg is rather mediocre: rambling script, nothing-special cast (apart from two Sir Michaels: Caine and Gambon), uneven change of scenes. The latter is most annoying as you lose track of events sometimes and well performed scenes vary with cheaply filmed chases or scuffles.
The biggest value of this film to me is the filming location - Saint Petersbourg - as I spent several years there studying just some years before the film was shot. Thus, most of places were familiar to me - well, that could be a reason my focus could transfer from watching the events into watching the background, this beautiful city.
If you are eager to see all Palmer-related films, then do it and include the film in question. Otherwise, you might feel bored.