Everything I've heard or read points to it being fairly effective for 30-06 and above. An 06 firing 180gr boolits can be pretty sharp, so a 30% reduction would be noticeable. When I was looking at buying one I talked to people who own the 7mm Rem Mag and 300 Win Mag versions and they say it makes a pretty big difference.

But for 25-06, 308, or 243 it'd be kind of silly unless you have a shoulder problem er sump'n.