My wife and I had not heard of Jamaica Inn at all prior to buying a DVD collection of Hitchcock's works. We decided to watch the film as the Jamaica Inn is near where my Cornish ancestors lived.
We actually found the film gripping. Yes, the acting is stagy at times, but we always enjoy the "old style" acting. Apart from the dodgy special effects, we found the film quite gripping. Others have said that the Squire was clearly the villain from the start. That is true. The tension comes from us being aware that this is so - and watching as the hero and heroine become ensnared by him. Similarly, the almost constant night time scenes and the howling gales are oppressive and eerie...
Critics may say that the film is not as good as Psycho or The Birds. However, this film was made 20 years prior to those films. I think that it was a good effort, considering that it was made on the eve of WWII.