Offline mode always works and you can always launch your games. The problem is getting into it. It'll often try to update Steam for 5 minutes before it gives you the offline mode option, but it IS there, even if you try to turn that mode on with no net on a fresh boot.
Edit: As it stand nows, my Steam folder is about 180gigs and my GOG folder 30 or 40. I always try to buy on GOG first to avoid ALL DRM, but Steam has been extremely pleasant. None of the nastiness I was expecting.
I get 'could not connect to steam' and that's that, nowhere to launch my games. If I lose connectivity while logged in it says 'game not available at this time'
If you let it do it's thing for long enough, restart the app every time it quits, maybe end task on it a few times, it gets through to offline mode. I haven't figured out the exact trigger, but it's perfectly possible and takes 2 mins of fiddling. A bit annoying.
Of course, if you're on the net, just hit 'go offline' and that's that.