We did practice drills with the I formation also and found that it does provide the other team with a different look. This is beneficial if you want to change gears when the other team is drilling cross court returns. You can essentially force the other team to hit down the line returns if the netman shifts cross court. This would be similar to the austrialian formation where netman is on the same side of server during serve. but with a more aggressive starting position.
As far as how effective it is, it really depends on how well they move together and how good the return team is. It is really just throwing in something different but usually a good team can adjust to it.