How have these affected you boats? Faster? Slower? Handeling?
What are the advantages or disadvantages?

The box puts the prop further behind the boat. Because of the transom angle and the attack angle of the boat when it is on plane, this puts the prop in deeper water. The water further behind the boat is also clearer. When the boat planes off, having the prop further behind the boat forces in into deeper water. All of these things allow you to run the drive at a higher height so there is less resistance. The greatest gain is to planing. A shaft height that is a couple of inches above the bottom when dry will still plane off with minimal slippage. The only disadvantages is that hydraulic steering is required. The rams will need to be installed on the extension box or have extensions mounted to the hull. There is also significant cost involved in installing a box because the engine, drive and transom assembly all must be removed.

Good info. I have one on my boat. I've been working out some bugs and just wanted to make sure I had the correct info on how and why I run a extension box. You forgot to add that they look cool. I ran a Imco SC drive 2 inch short with the Imco box.