Secret Agent might fall short of a true Masterpiece, but it certainly demonstrates the Master's touch. Many of Hitchcock's themes come together for perhaps the first time: espionage, murder, irony, mistaken identity, wit, allegory, moral ambiguity, etc...
The casting is great. However, Peter Lori played his part, the General, just a bit over-the-top for this movie; but since his character adds a fun-factor... what the heck -- let him go for it.
A definite "must see" for Hitchcock fans, classic film buffs, or anyone looking for a good suspense thriller.