I have been using this stuff for a while. My Boyd's wheels on my truck are not clear coated and this was the only spray cleaner I found that is safe for polished aluminum wheels. This should testify to its safety.
I also use it on painted, clear coated wheels and it does a great job. After some really spirited driving, the caked on break dust needs a little motivation to get it all off, but this is a great cleaner. Just a spray, a little help, and a wipe with a MF should impress.
I have never used P21S, but I think the price is too high as well. Hot Rims is only $3.99 on sale at Checker (Shuck's - Kragen).