I've shot my 3" DAO GP100, with Safariland speedloaders and a Blade-Tech paddle holster, the last two years at my local IDPA matches. I've won my division both times -- and last year I actually had a competitor!
But it's especially fun when you can beat the times of a few of the "bottom-feeders".
Other guys have done OK with the Ruger too, according to this article by Massad Ayoob
S&W may rule IDPA SSR, but it sure doesn't own it. Former Midwest champion Dave Maglio earned the coveted Four-Gun Master ranking with a Ruger GP100 traded in years before by the Milwaukee PD. I won the New England Regional in SSR in the summer of '05 with an out-of-the-box GP100.
I am still looking for better speedloader pouches. I've got an old 'Split Six' style leather double pouch, and and some generic velcro-flapped ones that I found on sale.
One thing I've learned is: when you fill up your speedloader, give it a good shake to make sure it's latched properly. During a stage, picking up loose rounds out of the dirt really slows you down! It's good entertainment for your buddies, though.