Lots of people seem to be have horror stories of doing instances with Pickup Groups. Maybe it's just Skywall, but--knock on wood--I haven't had any real issues with Heroic PuGs. I complain about things, but they're really minor irritations. So far, Heroic PuGs for me have pretty much been veni, vidi, vici.
The only interesting PuG happened a few weeks ago in Heroic Violet Hold while I was Ret. We wiped once on the Voidwalker boss. The second time, three of us died on the Voidwalker boss, but the last two killed it. Unfortunately, we ran back and zoned in, not realizing that you can't do that. So that attempt ended in a wipe.
The third time through, we start it up, and are on wave two when the boomkin announces, "Sorry guys, I don't think this group can do this" and hearths out. We're all like, "What the ... couldn't he have said something before we started!" So we four-man the waves and kill the Voidwalker easily. Thankfully we have a warlock, so at wave 10 we summon a DPS warrior to come help and we finish the instance cleanly.
So I dunno about Horror-story PuGs. On Skywall at least, Heroic PuGs are pretty good.