I've been lucky -- once my game has been activated, I have never had any real trouble w/ offline mode.
When Steam's servers were a bit ago down for maintenance, the day after I got L4D, L4D's offline mode for Single Player worked just fine.
I've never used Steam's back-up utility, but I usually just back up the Steam GCF's and the actual game folder (if there's anything in there; usually is for 3rd Party games). How is Steam's back-up utility?
Steam is far from perfect, but at least it has improved on a major gripe of mine. Now if only Valve would give an option of shipping retail boxes with every purchase.
That would be great. But, if there's a game update and you already have it -- and the game's been updated more times than you can shake a stick at and/or has gotten HUGE updates, you might wanna back the whole game up (so you ain't gotta re-DL the updates).
Hell, NBA 2K9 PC recently had a 1.5 GB freakin' patch over Steam! Why so big? That's b/c PC version got its first update -- which basically included all the new additional animations, dunks, roster changes, new players, improved AI, and etc that the X360 been getting through for Living Breathing Updates that occur every day; so of course the PC patch was gonna be huge!