Mostly I use steam client for watching porn.
#TrueStory.

1. Got answered many times now. So see above.
2. I have 2 Steam retail games (Skyrim and Deponia 4). If you install them from DVD it will install all data from DVD and download the rest (e.g. files not on DVD, patches,...).
The files from Skyrim are outdated on DVD so I have to download a lot more from Steam. Deponia 4 was a really small download after I installed it from DVD.
3. Backing up game files is really easy with Steam. I tried out both (just copying the game folders and using the Steam backup feature). The first one is much faster. Just copy and past. The second one can be used to create your own game DVDs or just a backup folder for an external drive (but it is faster to copy and past the game folder). Only one time i needed to download some files after restoring a game (Final Fantasy IV).
4. I only can say that it would be troublesome to stop the auto game updating. I don't know if it is possible to disable client updates. They are quite small and containe bugfixes and security improvements.
5. Nothing I can say here. I never used it. If my PC is running I am online anyway.

I believe I can answer some of the questions.

1. My steam folder in program files is 1 GB. Half of them is Web cache.

2. Sorry, never bought retail from Steam.

3. Yes. Backup feature of Steam is very easy and powerful, never had problem before.

4. EDIT. NO. I just checked and the option to disable updates is gone. Now there are 3 options. 1- Always keep this game up to date, 2 - Only update when I lunch it, 3- High Priority.

5. To be able to use offline mode you have to login once online. Then you can use it offline whenever you want. It will just ask for your password, enter it and thats it. But keep in mind that offline gameplay hours and online gameplay hours are separate.

EDIT: Changed #4 from Yes to No.

5 years ? and you think you have been using it more than me a user since 2008 ?? need to check facts you can literally zip the game folder , if you dont want to use the backup feature if the game is among those listed here
http://steam.wikia.com/wiki/List_of_DRM-free_games

you dont even need the client again .

and appmanifests are just 1kb files its not that hard to copy all of em

I have never been bothered with any steam updates with the way you mentioned it , client updates either when you reopen the client or notifies you of a update when online , offline mode is not something you have to select when online , when you start up the client and it fails to connect you can go in offline mode unless
1) the client crashed the previous run
2) you wiped out the cookies and history, cache
3) system crash when the client was on

what kind of software do you expect that wont check if all the files are corrupted or not in todays world ? so called "'uh, reinstall, verify integrity' stuff, check. " you mention

i dont know why you seem to think automatic updates are something painfully ? why do you think they are releasing gog galaxy ?

i have frequent power outages here and i have never had any issues going into offline mode and playing the single player games
x

Wow at the post.
First of all, I've said "I've been using Steam far longer than you think," so nice twisting of my words here. In any case, sorry that not all the games are as nice as zipping them for the future. That's not how Steam works. And let's drop the DRM-free list, it is irrelevant to this discussion. In order to play a game, you have to have the client, and in order to copy and paste the games from a machine to the other the proper, non-Valve way, you have to copy the folders along with the games' respective appmanifests. Want just a game out of a hundred? Good luck finding its relevant appmanifest.

Sorry, client updates work this way. When launching, boom, update in your face. When regularly using the client, your internet might be suckered and then Steam just goes up and say "Hi user, I downloaded an update just now without you knowing!" You are literally forced to the latest update no matter what you do, and if you attempt to object, you'll be exciled to offline mode on startup.

I expect the software to accept that a game exists simply by the folder of the game being present, and not by some appmanifest. Verifying integrity and whatnot can come later. This feature is not a deal breaker in 2016 as much as not needing the client is.

Did I say automatic updates were painful? To be honest, yes. I don't want to surf the internet, only to find my internet becoming slow because an update was initated without me knowing (besides the regular Patch Tuesday, which I can disable my going into a metered connection). I wouldn't like my games to have features taken out for new features with me swallowing my spit. Galaxy is there only to grab the audience who likes those stuff.

Sorry, but insofar, I'm inclined to say that offline mode is just a total excuse to their biggest problem, and even in that it fails. You need to login beforehand and you need to relog to the internet frequently to keep it in check. Of course, you can't uninstall the client if you want the games.

Whenever I bought a retail game that was bound to Steam, it always downloaded the whole Installation from Steam, the disc in such retail versions is just a dummy.

i can confirm the punlishers are really becoming lazy , i was about to purchase the retail box of wwe2k16 when i noticed that on the box that it contains nothing but a steam code :O aka its just a empty cardboard box you dont even get a crappy disk

and the game is a 50gb plus download ....

Hm, my post disappeared!
So again:
Steam always loads the full game fom it`s website, even when you start the installation from DVD. So the DVD is not more than a dummy.

Offline mode is still poorly implemented, and doesn't work consistently enough to be considered reliable. But, if you want to play... most interesting stuff these days, whether it's AAA, indie, or in-between the two, you really don't have a choice. GOG's sure as shit not pulling their weight with most genres.

It's only 44GB Downloaded.

1 - it's like a dick: sometimes small, sometimes big
2 - not really, but yes
3 - a way? why would you only need a single way? in deus ex 1 there's more ways to accomplish same objective
4 - you tell that to steam and maybe they'll give you cookie for good advice. i won't tell them because i'm fat enough already from cookies
5 - i have no idea what your talking about :-0 but thanks anyway maybe i'll look into it

still thats a ton considering my slow internet connection they could have at least dumped some of the data onto a disc

Just picking nits. You're right, it's huge. But the details were just a little off. :)

Thanks, I marked your reply as solution, since you provided some good, to the point answers to my questions, along with some useful extra info regarding Steam.

Thanks to everyone else for their effort, as well! There were many useful posts, but I could only mark one as the solution.