Wolf3: I've bought a ton of games recently and am way behind on downloading them. I can't recall ever having problems downloading stuff from Gog (note that I use my browser, I didn't like that Air download manager as it spewed millions of files instead of a handful).
But today I've had different games refuse to download more than 15-43MB of 1.5GB. My browser claims all 1.5 is downloaded, but the actual file isn't...it just stopped hours early.
I think GOG downloader client has had similar problems also in the past (like a year ago), considering that maybe 10% of my already downloaded GOG installers turned out to be corrupted (while all the HumbleBundle, DotEmu etc. games on the same hard disk were all fine, suggesting it was not a filesystem problem or "bitrot", but that the downloads were corrupted from the beginning.
I don't recall any of the downloads being originally reported unsuccessful, yet many of them apparently were. So if there is some kind of integrity checking when using the GOG downloader client, apparently it can't be always trusted because corrupted downloads are still claimed to be ok.
This is also the reason why I am disappointed by GOG removing the pre-installation integrity check from 2.x installers. Nowadays it is there only in the bigger multi-part installers, and unlike earlier, the game proceeds to install the game right away after the integrity check (which is not what I necessarily want, if my meaning was just to check the integrity of the installer, not to actually install it).
For the smaller single-part installers, you can right-click to the installer properties and check if the digital signature is ok (in which case the integrity is ok), but that is many more clicks away, much more cumbersome than the earlier double-click.
I want the universal integrity check from GOG 1.x installers back! And on top of that, some way to check the integrity of several GOG installers in one swoop, with the client or not.
That said, I downloaded "I Have No Mouth..." yesterday with both the browser and the downloader, both downloads were fine. Download speed was a bit slow for some reason though (even with the downloader, around 600kbps when it could earlier go up to 8Mbps or so I think).