The downside is, when they partner with a company like Ubisoft (who's been rather naughty with DRM these days), the partner gets to dictate the terms. So before I buy a game off of Steam that isn't a Valve title, I always check the fine print to see if it's going to use Macrovision or SecuROM (with no disc? Seriously?) or have one of those "you must be connected to the Internet at all times or we'll call yer mom and tell her what a pirate you are" rules.
Skyrim's my pet case lately: my desktop syncs with the Cloud and uploads my screenshots, while my laptop can play the game with no Steam involvement whatsoever: if I connect to Steam or I don't, it works. I love the way they've published the Skyrim creation kit, and how easy it is to "subscribe" to user-created content that it'll drop into the world and keep it in sync as the creator updates it.