Not having to worry about keys and updates is fantastic for me, plus good discovery and upselling opportunities for developers.
It's not that great for me. I've had a bunch of my Steam games put out breaking updates and there's nothing I can do about it but wait for subsequent rollbacks. It's a major flaw in the system that a breaking update can be forced down users' throats.
This includes breaking updates to the Steam client itself - which stopped working on 10.6.8 in November. I can roll back to an earlier version, but it immediately auto-updates to the latest version.
So now I can't update any of the games I've bought through Steam, or reinstall any I've removed.
Obviously I can't buy any new ones, either - but, even if I eventually update OS X (breaking many older Apps I depend on for work) and am able to run the Steam client again, there's no way in hell I'll ever buy a game through Steam again.
Automatic updates is great. The lack of ability to easily rollback is not. GOG's Galaxy has rollback built in. I feel like Steam will add it eventually.
It's not that great for me. I've had a bunch of my Steam games put out breaking updates and there's nothing I can do about it but wait for subsequent rollbacks. It's a major flaw in the system that a breaking update can be forced down users' throats.