I have not used any of the download services. Do they let you download an ISO to burn, or is it an online installer?
As far as I know, Impulse, Gamersgate and GoG all definitely let you archive your game easily and you can install your archived game on any other PC unless the publisher of the game decide to force a special DRM on it that requires activation/deactivation.
D2D is the same for the most part.
One thing nice about Impulse is that they are starting to offering games with their "GOO" "protection" which actually let you resell the game! But right now, there is a problem from GFWL which could nullify the resell feature of GOO, since if you decide to log on to your GFWL account with the game, it will ask you for the game's serial number and thus tying the game to your GFWL account, which make the game "un-resellable." (Actually, you could resell the game, but doing so will definitely make your buyer very unhappy, should he decide to try to log on to GFWL).
Here is a quick run down of the currently well known DD services, in order of my preference (someone who doesn't care to stay online all the time just to be able to play purchased games, who rather enjoy his game w/o being "connected" to some online community, that is outside of the game, all the time):
1. Tie between Impulse and GoG.
GoG should be everyone's #1 stop for classic older games, period.
Impulse:
Pro:
- No DRM, (unless the publisher forces one to be used, *cough* Anno 1404 *cough*) besides needind to download the impulse client to download the game for the first time. And as a result, there should be very little problems when it comes to modding and patching games purchased from them.
- Great customer service. Very responsive CSRs and for Stardock published games, they even offer refunds.
- Good selection newly released, higher profile games.
- Easy backup and and installation of games/software on different PCs w/o the need for impulse client (with the client it would make the process more mindless. Backups are just zip archives, which I usually extract before I burn them onto a CD so that I don't have to worry about whether or not the target PC will have a program that can extract zip archives.)
- Very good download speed (YMMV)
Cons:
- Game selection isn't as extensive as Steam.
- Sometimes new games don't show up until days after they are available on steam.
- Not as "connected to the community" as Steam. (But to me the level of "connected-ness" is just right -- you are not required to be connected, but if you decide to connect to the community, there is one waiting for ya)
- Don't have as many crazy deals as Steam.
2. Gamersgate:
Pros:
- Like Impulse, very little use of restrictive DRM unless forced by publisher. (Again, Anno 1404...) There is no "over-all" client that you need to get to download the game, you download an .exe file for every game you purchase, and running it will download the game.
After the download is finished and the game is installed, the executable will ask you if you want to "delete the temporary files" choose NO, since the temporary files are your backup of the game. Burn them on a CD if you like.
- Better (than Impulse) selection of new high profile games. Esp. games from Europe. (I find that Impulse and Gamergate complement each other very well in term of the selection of games.)
- They just started to offer pre-order "pre-loading" of games. Which means you would be able to download your game before it is released, but you won't be able to play it until the release date.
Cons:
- Less newcomer friendly. (Such as automatic patching)
- No "steam-like" online services (chatting, calender, groups etc.)
- Need "Gamersgate specific" patches for most games.
- Again, compared to Steam, Gamersgate offers less deals, and less/slower availability (on avg) of new games.
3. D2D
I haven't used them much until now. They used to use their own DRM protection on every games they sell, but lately it seems that they have abandoned that practice.
Steam (the "grand-daddy" of DD services):
Pros:
- Most extensive "community services." Highly recommended for people who want to be "connected" at all times even when they are playing strictly single player games.
- Most extensive collection of games. They usually have all the new high profile games on the release date.
- Usually have very good deals.
- Most newcomer friendly.
Cons:
- You have to run the Steam client whenever you want to play your game. Which means you will need an internet connection. However, you can configure Steam to operate in offline mode. But it will let you know that it is not happy in that mode...
- Need "steam specific" patches for games. Also, there are more reports of steam purchased games being unfriendly to modding.
- Annoying popup notices for group events in your calender. You disable them (as far as I know) w/o leaving the group... These pop ups could alt-tab you out of some games.
So to summarize:
If you like to enjoy your games at your own pace, w/o distraction from others:
Then:
Use GoG, Impulse or Gamersgate
If you check your facebook, myspace, etc. accounts all the time, and twits (sp?) regularly
Then:
You will love Steam.