Holy crap just got my HM5 pads and the o-rings in, the S500's feel so plush and lightweight now! Easily one of the most comfy setups I've ever listened to, especially with how painful the stock pads were... Noise isolation is also slightly improved. Also helps improve its narrow soundstage, which was what IMO was the S500's biggest weakness.
Welcome to the elite S500 HM5 club!!! You're officially V.I.P. now.