They have an anti-large bonus on their range attacks and good ap-damage. With the current Skaven roster that role is already taken by the Poison Wind Globadiers. Jezzails will thus be very good when shooting large targets, especially heavily armored monsters and heavy cavalry. In the tabletop they were one of the best armor piercing units in the entire game so in Total War Warhammer these guys will probably have very high armor piercing damage as well. They have an extremely long range which I expect to be something like the range of Ushabti Great Bows.
Jezzails are basically Skaven sniper-teams that shoot a (very) long rifle. So with the new DLC we all highly expect CA to implement one of the most iconic Skaven units into the game: The Warplock Jezzails.
