It lowers the rear of the transfercase (pivoting on your engine mounts) which causes an increased angle on your front driveshaft and put additional stress on your engine mounts.
I think you'll find that the majority vote will be to not install these but rather correct any driveline issues by doing one (or all) of the following:
1) improve your pinion angle
2) clock your transfer case if necessary
3) use the appropriate u-joint for whatever angle you are trying to go with.