Firek: As far as I can tell from our logs, it's been a long while since you last contacted us, though I understand that you're basing your opinion on other forumites' experiences. Trust me when I say that we have, in fact, scaled up considerably. :)
Yes, I haven't had any major problems in a long time, but threads do crop up from time to time where people haven't heard back from you in a long while. Naturally, the impression is skewed somewhat by the fact that most people who do hear back from you in a timely fashion rarely post about it on the forum.
Wishbone: Still, there have also been numerous cases where people have not had a reply from support for weeks or even months after they posted their tickets.
However, when the problem is related to payments, GOG is usually very quick about getting it fixed.
Firek: Nobody's perfect, and while there must have been many such cases over the scope of 5 years (linkedIn recently told me that's how long I work here:O), but I would say that the percentage of these is really tiny, compared to tons of regular tickets.
There will always be cases when we can't reply for a week or two, if the case is exceptionally convoluted, like when the answer involves multiple departments, or legal stuff, or if we have to wait for a developer's or publisher's response. The best thing to do in such situations is to tell a user that it might take a long time to get an answer, but on the other hand we can't always be sure IF we can get an answer in the end.
Of course no system is perfect, and as I said, you're certainly doing the best you can. Still, as you say yourself, keeping the user informed of delays is the best thing you can do in those situations, and I think that might be where you could improve your process. I know the problem from my own work. When you're working on a helpdesk case that is very complicated and takes a long time, writing to the user at the other end that the problem still isn't fixed but you're working on it isn't the foremost thing on your mind.
Another thing you could do is to bring back the support tab on the account page that you removed more than two years ago in order to "fix a bug" ;-p