It helps that Steam doesn't need to validate games every time you play them. I think it validates them once a month.
It looks like Valve finally managed to iron out Steam's inability to switch to Offline mode if an Internet connection isn't present when Steam starts, too... so you should be able to play installed games if your connection goes offline.
There have only been a minor handful of widespread issues with Steam's system, the most notable being Half-Life 2's launch... in 2004.
Now, if only they could invest in better infrastructure for their Friends service and matchmaking systems (affects TF2 MvM, CSGO, and DOTA2) so they wouldn't go down 2-7 times a week...