My opinion;

My slightly more illustrated opinion;
I don't like Steam. It treats the customer like a criminal; you have to ask permission to play a game even after buying the physical media (and let's not pretend online-verification is any less crackable than CD-keys), and effectively you're paying to 'lease' the game from Valve; it's tied to a (in the EULA anyways) non-transferable user-account which means you only 'own' the software so long as Valve supports obtaining and activating it, prevents you from being able to re-sell a good you own as second hand, and leaves you dependent upon memorising your password and the activation server working should you wish to uninstall/reinstall the progam.
Also, it's an unecesary program, which sits under HL2 and uses up resources it simply doesn't and shouldn't need to. I don't need a piece of memory-hogging communications middleware to play a single player FPS! I'm uncomfortable with it being able to detect and add other Valve games, because the idea of a program with internet access doing so seems dangerous even if it's not malicious in intent.
Although one of the benefits of Steam is the provision of patches / updates, in my experience this doesn't work well in practice. I've had frequent errors using it to try and perform (lengthy) patches, having to repeatedly restart the download process; this is far inferior to having the option to download patches in the 'normal' way. Also in usability terms Steam is a bit like a trojan; whilst they belatedly added an ability to stop it connecting, if you update it in any way it seems to automatically reset itself to start on system startup, even when explicitly against the settings I've gave it. That is very sloppy IMO.
Steam has stopped me from re-installing HL2, because IMO it's not worth the hassle to do so. For me, that illustrates the key problem. It (digital distribution) is perhaps a positive move, but not implemented like this. albiet it's worth noting that - and this is established in UK law, although i don't have the exact citation handy - digitally provided software has a far lower obligation with regards to quality, support etc than that provided on physical media.
Now, I've actually praised Stardocks equivalent before. On a functional level, it's not
that far removed. But IMO it has a few key advancements, the key ones being that it doesn't need to chug away in the background when a game runs, it doesn't decide to scan your PC for other matching games (AFAIK?) and that you don't need an internet connection to play, for example, GalCiv2 (just a CD key). Plus it seems more reliable, less resource intensive and is IMO far more user friendly. The one problem it does share, I think, is the reliance upon a central server (albeit to DL updates rather than play) and the inability to manually download game patches. But it's interesting for me how 2 essentially functionally identical approaches have given me vastly different experiences. Steam was negative, Stardock positive.