You're talking about their windshield wash booster right? I have been using it all winter and have not noticed any damage to my paint. It does cut thru the crap on the windshield pretty well, which has been a big job this winter!
In fact I gotta go put some more in right now! Empty again!
I've been using the P21S wash booster for 2.5 years now. Haven't noticed any ill-effects from it. To echo what others have said, it does a terrific job cutting through winter grime. I even added some to my girlfriend's car and she remarked how much better it worked over the standard factory solution.