I first used Impulse (under new Management now with Gamestop) to purchase Sins of a Solar Empire. I downloaded the installer and the game was installed quickly. It was a decent first experience. Then I looked to the bottom right of my screen and saw a nice little icon for the Impulse interface. No big deal. I had to recover my account at one point. Through my own stupidity I had forgotten my account login information. The restoration process was quick and painless. I shot an email to Impulse support and got an email back. The interface was clunky but dealable.
Good old Games is impressive. The interface is web-based so there is no bloatware installed to your computer. It's clearly listed on the site what operating system each game is compatible with(take the time to read that). They recently offered Empire Earth for free and do free giveaways often, Fallout 1 and 2 recently were offered as well. At the same time as offering old games that you cannot find in the store GOG takes the time to offer new releases to stay competitive with other services, hint: Alan Wake is still 15 bucks.
I've used Gamergate a few times. This site, like Good Old Games, uses a web interface as well. When you purchase a game you'll get a small amount of Blue Chips. Every game has a US dollar cost and a Blue Chip cost. It's like a Best Buy Rewards Card! Eventually, after multiple purchases, maybe you can buy something. It'll probably take a while if you are like me and only jump on sub-10 dollar deals but at least the options are there and gives Gamergate a unique feel.
I have not used Greenmangaming yet but i still wanted to incude a mention of them as they have a unique concept over there. In effort to go Green you can trade in your digital games for credit towards another purchase. A revolutionary idea in my book. The prices fluctuate daily according to demand and aging of the game.
Amazon digital distribution service had a shaky launch in my book. The first game I purchased was extremely hard to get installed. The downloader automatically starting downloading to my main small SSD drive even after I had selected my secondary drive as the download location. In the end I had to uninstall the software, reboot, and reinstall it to get it working. I had to do this with both of my first purchases; however, since then the service has solidified and become the second most successful digital distribution behind the elephant in the room...
STEAM, look for part 2 in the coming days.