The Summar was Leica's first high-speed lens, introduced in the mid 1930's. Up until it's introduction the fatest 50mm lenses that Leica made were the Elmar3.5/50 and Hektor 2.5/50.
The Summar has quite high resolution, but fairly low contrast and it is prone to flare. It has a unique look in the Leica lineup and really shines for black and white work, if you are looking for that period look. It can produce very beautiful results.
From the factory the Summar came uncoated, but you may encounter coated examples that were later 'upgraded' . There also is a rigid version, but it is very rare and expensive. The front element is very soft and it is close to impossible to find a mint example.
The Summitar followed the Summar, just before the outbreak of the war and in technical terms is the better performer. The Summitar performs a notch below the collapsible Summicron. Coated Summitars were restricted to the German Armed forces during the war, but it is not uncommon to find these. It's a good lens if you are looking for a period look. It does not perform at the level of a modern lens.