As far as improving vision, the lenses did quite well.  The type of plastic used was easy to mold, inexpensive, and polished to a smooth optical surface.  The lenses were sturdy, and could last for years with proper care and use.  They could even be tinted slightly so the patient wouldn't lose them if dropped.
The very first contact lenses were actually developed in Germany in 1927.  They were made entirely of glass and covered the eye completely.  By 1937, a new type of plastic was used instead of glass, but the contacts still covered the entire eye.  This was the case until the 1950's when lenses were developed that only covered the cornea itself.
Above: A contact lense from the 1930's.
Thankfully, these old types of hard lenses are essentially obsolete and are usually not recommended by eye doctors.
But there was a serious problem.  These lenses, specifically the material they were made of (polymethylmethacrylate, or PMMA) could not properly transmit oxygen or carbon dioxide. For the cornea to be operate effectively, it must be able to take in oxygen and release carbon dioxide.  Hence these lenses were extremely uncomfortable for the user, and in addition required special solutions to help water and tears to adhere to their surface.
Soft Contacts: Daily Wear
Soft Contacts: Extended Wear
Rigid Gas Permeable Contacts
Disposable Contact Lenses
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