My local homebrew shop carries malted oats, and I use them pretty liberally in my brewing. I've brewed with oats for many years, but the past year is the first I've had malted oats available locally. In my experience, everything you get with whatever unmalted oats you've used in the past you will get with malted oats, plus a little more malty flavor. All my porters and stouts absolutely get them as well as many of the Belgian styles I brew. I'm sticking with malted oats too now that I have a reliable local source. As for an amount, I go with 10 to 20 percent of the total grain bill. Most are closer to the 10% range, but I recently did an oatmeal stout that malted oats were 25% of the pale malt.