Secondhand experience with Steam, here. Friends of mine bought a Steam-powered game, couldn't get it installed on their machine (even with my help, and I like to think I can did my bit to cajole PCs into doing such things, I'm certainly no slouch with the whole business) and even after getting a replacement set of disks from the shop. Anyway, they sold the game on (media, packing, all relevant activation codes) through eBay. The recipients apparently tried to install it and got warned that they were using a pirated game.
Now, I can hear the gears grinding, and yes, Friends had probably managed to register their copy properly at some point, despite it not completing its installation, but as it didn't play with some aspect or other of their machine (specs being Ok, because I checked, but maybe a minor incompatibility of drivers or something, or problem connecting with home[1] if only it'd given some halfway sensible errors on failure) it was totally removed and upon re-sale (lock, stock and barrel, which should be a fair-use transaction, although not all T&Cs agree) renounced. Yet still there was a fuss from Steam about it that AFAIK never got resolved to anybody's satisfaction.
Of course, there are probably many stories with perfectly satisfactory endings. If I could even remember the details, it could be an even better anecdote but still one of the few, rare tales of woe out of millions of success-cases.
From a personal POV, I'm just generally not too happy about on-line registrations, either, although I appreciate the need for them to avoid the most rampant piracy issues (where the main part of the game involved isn't already the on-line connection for PvP gaming or character sharing, where licence checking could be performed on such occasions but the standalone install doesn't really need to ping servers every time it starts up in single-player mode), and as such I'd probably be likely to avoid games with Steam registration/etc mentioned on their packaging, even if I had time to fully immerse myself in them and be a gamer-head in any big way.
But as a datum point, I hope you find the above informative. Again, emphasising that it's a single hard case that I know that probably isn't representative of anybody else's likely experience.
[1] From what I could tell, the traditional installation media was augmented by download. I know there's download-only Steam purchases, but while I can't remember all the details, I've a feeling that this was two DVDs (or so) and still needed to download more, and not just do a quick licence checking...