Most of the games I have bought here, I already have on disk.
To me it's worth the ridiculously small price tag to:

1) Have a permanent backup (flash drive & redownload, just to be safe).
2) Get them runing without the hassle of having to fix compatibility issues myself.
3) Run them on my netbook that doesn't have a cd drive.
4) Safeguard against my kids screwing up the disks.
5) Encourage GOG to release some of the classics that I dohn't currently have.

Short answer: yes.

Long answer: yes, because of convenience, and my original boxed copies will be better preserved. I still collect physical copies of games in many cases, but the less I need to use them, the better they will look in 10-20 years.

EDIT: better answer: see Rodzaju's post (post 16).

yes. i for one just by them because :
1) i'm lazy and the gog versions are hassle free
2) i h8 to have the disc in the drive during play
3) a disc is a disc and updates have to be installed after a while

i love them boxes sitting on a shelf so i can watch them during load screens :)

Yes, just yesterday I did, actually. I bought The Witcher (I already had the boxed version) and I also got Gothic. I find it more comfortable to have digital versions of the game, and besides I want to support GOG's business model.

Same here... also i'm not scared of doing a bit of hack & fix for compatibility, if i already own a physical copy (AND there's a fix available....)
So, for me, it boils down to buying only really incompatible games resurrected by some GOG magic, or games that i've never played before.

I bought all Fallout games - I have them all on one DVD, but in Polish version (I am kinda allergic to game translations - somehow, they always screw something up). So, for convenience and game experience, I re-bought'em all.

Same story for Planescape:Torment - I have a Polish 4-CD version.

Oh, and I had Moto Racer that came bundled with my ancient Matrox graphics card (somewhere in the 20th century). No way to run it on a modern machine. Bought it mostly for the nostalgia factor.

And I also bought Heroes of Might & Magic 3 while I still have a Polish 2-CD version (one of the two disc is scratched - I could have it replaced, but why bother when I have GOG?).

And I also got Jagged Alliance 2 here (I own a 2-CD version with a pretty atrocious Polish translation).

I had M&M VI that came bundled with some magazine a dozen years ago, but I got the M&M I-VI bundle anyway. I used to play M&M III-V (pirated version) back in 1990's, and World of Xeen was my absolutely favourite RPG (it still is my 10/10). This was an opportunity I simply could not miss.

I buy games on GOG that I already own in a box for multiple reasons, the most important one being to avoid hassle. It's a hassle to setup Dosbox, it's a hassle to install off multiple discs, it's a hassle to disc-swap in games like XIII. A GOG install is quick and completely painless. I might also buy a game on GOG to get rid of annoying DRM on a boxed version.

That said most of what I buy here is either games I missed when they originally came out or games I used to own but sold. I went through a "I don't play videogames anymore" phase from 2003-2004 and sold all my old boxed games on ebay. I have been buying them back ever since. GOG gives me a way to buy some of them cheaply when the old box copies are too expensive to buy right away (though I do want everything in a box eventually).

I would say 90% of my GOG collection consists of games I've purchased previously. I absolutely cannot stand using physical media for games any more, to me it feels like a throwback to the 90s, guess there's an irony in that somewhere.

I'll probably buy the fallout games as fallout 1 doesn't seem to work right ton my system atm.

I also have the 3rd inredible machine but bought gogs pack anyway, and psyconauts, but on 360 XBLA, so i thought since I no longer can use the 360, I'd gog it...

I try not to re-buy games unless it's necessary. Ex. my disc/version won't work on Win 7, needed to complete a promo deal or it has a bonus item or expansion or something that I want etc.

I just find that because of the prices here I don't have to worry about it too much, but I would rather buy something that I don't have any copy of first.

I've been playing PC games since the time that PC games first became available. I've purchased many (many!) games from GOG that I still have the original discs for. I do this for several reasons. First, I like being able to simply click on an icon and start a game up without searching for a disc. Second, I like being able to play old games on new operating systems (this means that I don't have to keep a lot of old hardware around). Finally, I've bought a bunch of games that I know I'll never play but I want to support GOG and what they're doing.

For me, the holy grail of publishers is EA. I always seemed to really like their games (not their business practices mind you...). Once EA (and LA) are in the fold, and I think this is extremely likely hopefully in the near future, I'll most likely buy every game released. I can't wait!

Yup I've bought stuff here for compatibility ease and to support GoG (plus get extras) that I either have an original physical copy of or have on Steam already....

A rough list that comes into this category is:
Rise of the Triad
Duke Nukem 3D
One of the Earth games (have all three on GoG... forget which I have the physical copy of)
Commandos
Commandos 2&3
Syberia
Syberia 2
Caesar III
Civilisation: call to Power II
BloodRayne
BloodRayne 2
Psychonauts
Beyond Good & Evil
The Longest Journey
Evil Genius
Alien Nations
Empire Earth
Postal2
Stronghold
Ghost Master

Hmm.... quite a bit of my collection come to think of it... heh.

Well, yeah. I'm actually on a kick at the moment about repurchasing old games I own on disk on digital delivery services. Both Steam and GOG have been incredibly useful in this regard.

Though I will admit that my BG2 repurchase was more pragmatic than anything; for some reason during the period I was buying up Interplay RPGs their disc mastering process seemed to be horribly poor. Three of the four original game disks have, over the years, simply stopped working in any drive over the years (And the same thing has happened with IWD, but I managed to get a new copy of that in a IWD Complete bundle literally one month before Interplay's collapse).

I have never bought a GOG and then purchased a hard copy. However, most of my GOGs are games I've had a hard copy of at one time in the past.

Most of the games I've purchased I also own on disk. You can't beat the convenience of having a digital source that also primes the games for modern OSs. I also usually wait for stuff to go on sale, so the price can't be beat either.