If I had NOT known this was a Manmohan Desai film, I would have dropped the rating by 2 points. I have come to enjoy Manmohan Desai films as a mindless entertainment. In 1970s when I saw his films, initially I had hard time appreciating his films. They seemed incredibly silly. In spite of this, I kept on watching his works: Dharam Veer (1977), Chacha Bhatija, Parvarish, Amar Akbar Anthony, and Suhaag. But by the time I watched Naseeb (1983) in theater, my brain had become immunized to preposterous coincidences, incredibly resourceful villains, and 'brawny-no-brains' heroes. I actually had enjoyed Naseeb. I think Amitabh Bacchan had a big part for why I continued to watch his films.
I had not seen Sacha Jhuta back in 1970s. I happened to watch it on DVD forty years after it was made. My immunized brain tolerated the story -- I even almost enjoyed it. This movie had plenty of 'coincidences' and villains smarter than the hero. In this movie, the hero starts out as a moron and changes into a smart guy. Only Manmohan Desai could make such an extraordinary transformation possible. Desai had always some interesting twists in his story. This film was no exception.
Also, his movies never had outstanding songs. However, there were always a couple of popular and enjoyable ones. Again, no exception here. Rajesh Khanna produces his trademark acting. Over the years, I have watched many comedian performing a mimicry of his dancing style. It was fun to see the original. I consider Mumtaz as one of the good looking actresses of her time. She looked better in some of her other movies, though.
Not much scope for acting in this movie. However, interestingly Rajesh Khanna won Filmfare award 1971 for it. I rate this film at 7, which includes 2 points for the nostalgic value. If you are a younger generation, probably you might consider this at 5.