Typically a bad idea.
Unless you know what you're doing and buy quality kit then you will likely ruin the ride and impact the handling. I know that manufacturers build cars to a compromise but they spend millions testing these cars to have, for the budget, the best ride/handling compromise on the widest variety of road surfaces.
You may, by lowering and (more than likely stiffening), mildly improve the handling (with suitable geomtery changes) on billiard table smooth roads (like those you see all over the UK
) but you will likely ruin the ride
and handling on real world roads.
Also with those blocks that raise the car to stop it being damaged on the transported... they have been known to be left in there whole (or in part)... so well worth a look when you pick your car up, especially if it looks like it is riding a little high...