On the plus side, their aft position keeps the sheet from rubbing on the hatch rails. On the minus side, they add friction and the sheet takes up more of the cockpit, making it hard for crew to move. So, you still need genoa blocks you can adjust fore and aft. The set-up I had was a standing block mounted on tracks. To trim the sail I would go cross cockpit to the winch on the windward side.
Some boats have a fixed cheek block that routes the Jenny sheet to the winch.
They can't replace the adjustable blocks, because the ability to adjust the genoa lead position is crucial. On the plus side, their aft position keeps the sheet from rubbing on the hatch rails. On the minus side, they add friction and the sheet takes up more of the cockpit, making it hard for crew to move. The set-up I had was a standing block mounted on tracks. To trim the sail I would go cross cockpit to the winch on the windward side.