Aside from a god-awful opening tune sung by Lewis (ugh!), "The Sad Sack" is an entertaining little film. Fresh from his breakup with long-time partner, Dean Martin, the studio instead gave him David Wayne AND Joe Mantell to fill the void. And considering that there isn't any of the usual singing, things worked out just fine. However, don't expect huge laughs from this one--just a pleasant little army comedy.
Lewis naturally plays the Sad Sack--a soldier who can't seem to do anything right. However, a dopey female Major takes him under her wing and gets a couple soldiers (Wayne and Mantell) to buddy up with Lewis and help him through basic training. Eventually, the trio is sent to North Africa where they have a series of mildly interesting adventures.
Like I said above, this is not a hilarious film but a nice little comedy. What I especially liked is that Lewis was far more likable and restrained than usual--without the excess mugging that sometimes ruined his later films. What you're left with is some nice acting, a decent script from this little time-passer. The only negative I noticed is that there are quite a few sexist remarks about the major--a product of the times in which it was made.