I've used them and didn't have any issues in the past on a road king. I have heard that the changing the shock angle can have effect on the handling but again I didn't notice.
I've had my lowering blocks (LA Chopper) ones for about 4 months with no issues just wanted to make sure they would work with the Harley premium shocks.
The style lowering blocks I have wouldn't work with the HD premium shocks. the bottom of the spring was too low an hit the lowering bracket. I picked up some 12" premiums an I think they are a nice improvement if you get a good deal on them...under $250, I got mine for under $200 shipped. A lot of people say the run the lowering blocks for thousands of miles with no problems an some say they can break an cause a dangerous situation an they alter the geometry. I might try to use them with some spacers an longer grade 8 bolts to add some clearance.