Chaos Concepts released a version a few years ago. UFO Extra-Terrestrial. They are now working on a UFO-ET 2.
I don't know about Jars, but things I liked about X-Com were:
-LOS/Fog of War was really well done.
-Jumping off roofs (LOS) to avoid getting killed by aliens.
-Lightness/darkness vision modifiers.
-Multiple positions for your soldiers.
-Enough maps/ships to keep the game interesting.
Many more. Overall it was a well developed game.
