I definitely thing the LAW has some great uses, especially when you have a team of engineers attacking a target, the combined attack can make short work out of a vehicle.
However, I the fact that the Law always comes down on top reduces it's effectiveness when you are fighting a tank and you have no support (which happens more often then not). It takes a lot of shots from the top to stop an MBT (or a lot of other vehicles), giving the tank a lot of time to find and kill you. There have even been times that I have failed to kill a vehicle after successfully hitting it with 3-5 LAW shots.
Therefore, I have found myself using the RPG as of late. The lack of tracking does make it harder to hit a vehicle (though the LAW detection is very small) and you have no laser designated tracking, but the ability to hit a vehicle in weak spots helps a lot. It also allows you to cause mobility impairment if you hit the enemy vehicle right, making subsequent shots easier and reduces the enemy vehicles ability to fight back.