Lowering blocks are subjective, they work in a pinch. Although, you are better off going with different sized shocks.
Shocks are affixed at two fastening points, giving you a solid foundation to the swing arm.
Lowering blocks are able to loosen up and pivot on there mounting point, which in turn can and will cause slop in rear suspension dynamics. Let alone cause some pounding on the swing arm itself, which can cause fractures in the material of the swing-arm.
Search the web, as there are pictures of the damage done by these blocks.
I tried them for about 2 months to make sure I really wanted my rear-end lowered, then I went with Burley<sp> brand shorter shocks for a permanent modification.
I ended up giving the blocks away to someone here on the forum, so he too could try them and see if he also liked the function of a lowered rear-ended suspension.
Too each their own, just remember, that band-aids are just that. They are temporary and not permanent modifications.