I bought a Civ III compilation a while back from D2D. It was my first digital distribution game purchase. I've held off on them for any other purchases for the following reasons:
* Patches and content updates released after the version you've purchased for the games I bought or was looking to buy were either very delayed, on the order of *months* (or in the case of Civ III, never materialized afaik).
* User created content, mods, and addons may not be compatible with the D2D version. This depends upon the game.
* D2D, at least from what I remembered with Civ III, loads an installable file system and/or pervasive DRM. I don't recall the details, but I do recall being happy to uninstall the game, then hack at my Registry to remove whatever D2D installed.
Bottom line for me is that there are other (imo, better) options available for digital distribution. None have been perfect, but I'd recommend Good Old Games (gog.com for old, but highly-rated hits), Impulse, and Steam. Probably in that order. Why not Impulse you ask considering the forums I'm on? Because patches do get delayed (not as much as D2D), and at least one game never got the latest update from the developer (Hacker Evolution, in my case, though that's also on the developer... who emailed me a separate updated registration key/download but never pushed that update through Stardock). Steam comes at the end of my recommendation because despite their game selection, they use modified executables and do something wacky with the file system - so some of my purchases weren't completely compatible with custom content created by the modding community (X2, X3, Railroad Tycoon series) and didn't contain the full set of modding tools that shipped with disc versions of some games (Railroad Tycoon III).