Ok,
I've just spent the last 24+ hours trying to download three freaking files. First I tried direct with firefox. Got to 98% and then said that the source couldn't be read and aborted. When I tried to retry it wanted to start over. It had taken hours because the download was only going at 60-80KBps on a 4Mbit DSL.
I said fine, I'll try in download manager (wxDownload), it got 250KBps and was doing great... for 10 minutes. Then gog.com (actually their mirror in San Fran bitgravity) started aborting my downloads and giving me 403 forbidden errors. So I tried AGAIN, this time using 1 stream only. Same thing after about 20-30 minutes.
So finally I tried gog.com downloader. This got to 88% and then gave me some lame error and said "sorry" then it removed the game from the queue. Again, when I re-added it, the download went to 0%.
I've sent two support tickets over the last two days and got no response. Guess they don't work or have any staff for weekends. And before anyone says it, yes I set it and go to bed. Problems is without babysitting it, the download aborts in a few minutes and the server starts giving me 403 forbidden errors. When I then have to log into the site, and click the link again to get a different source address.
But this isn't the first time it happened. I also bought a game several months ago and had problems with downloading. I had to download the files several times because of CRC errors from installer. Even on the 2mb part ones. I've since moved across the country and live less than 300 miles from the host servers. Still having problems with them.
Is it just me or does their download horribly suck?
P.S. Yes I'm ranting, I wasted a weekend trying to download something that should have been done in an evening.

That's too bad. I use IE or Firefox for all of my downloads. On GoG, I consistently pull about 20 mbit of my maximum 35 mbit.
In another similar thread, a poster referred to this: https://addons.mozilla.org/en-US/firefox/addon/201.
Maybe, that program will work better for you? I never use any download managers, so I can't really speak for its quality. Currently, the statistics show over 31 million downloads and 705 reviews with an average rating of five stars.

It's true, the support staff doesn't work weekends. They're usually very helpful though. As for your problem, I'm afraid I have no idea. I've never had any problems downloading from here, and I have about 30 games. Never used the downloader though, I always download everything directly in Firefox. Since it is now monday (at least where I am), the support staff should be showing up for work soon. They usually reply within a couple of days. Remember, it's a small new company. But they're extremely helpful.

Has anyone else used a download manager that splits the file into different parts?
Maybe their servers are messed up, but 403 is a server side (not client) issue. But even still, a single stream with one part shouldn't cause that problem.

I'm really sorry you're having problems. I'm sure support will be able to help you.
To answer your question, i never had a single problem with downloads (30+ games). For most of them i used the gog downloader and i never had any problems. I downloaded some of the smaller games with IE8 and not a single problem there aswell, so i really can't see anything wrong with GOG's downloads.
Edit: Never used a download manager for gog files

Yes, it's just you.. Maybe. But that doesn't mean you should be ignored.
I've never had any problem with downloading from GOG and pulling out the whole 10mbit I have.
I just use my browser to download the games and it seems fine. Never had anything bizarre happen.
You are best just talking to support about it.

Never had any problems with the downloads either, directly through Firefox and have never had a corrupted download.
Make sure you have no Firewall and/or Antivirus software in the background that might interfere with the download. This might include interference which causes the download servers to misidentify your HTTP stream and hence block you. If this has happened in the past and you're using the same computer / connection then that might also explain it as I expect the secury policy around the download server is pretty strict.
It may also be a temporary problem with the download servers.

I actually always use the downloader, and have had no problems at all. The issues your having might be to do with firewalls or such things, as you didn't mention if you've tried turning them off while downloading.
As a test I just left the downloader running for a while and I'm getting 500 KB/s, with no drops.

Agreed - contact support.....
I've used both regular HTTP download and the GoG downloader many times successfully for the games I've purchased (and their extras!)....

I have occasionally experienced minor slow downs in the downloads, both with the browser or GOG's downloader, but they always traced back to something on my end or on my ISPs end (stupid "connection has been re-set" errors), never to GOG itself. Corrupted files have never been an issue and I haven't used a download manager since I got off of dial-up internet access years ago.

Wowzers that's not happened to me since my old 486 with its 80mb HD

I've also never had a single problem. I've only downloaded 13 games, but many of them were the larger titles. I consistently get about 7 Mbps, which is what my connection is rated at.

I have a crappy 40GB disk. It's really too little space for these days, unfortunately i can't upgrade since IDE hard disks have pretty much dissapeared from the stores near me.

As an update,
Support did get back with me this morning. They had me run a test on their provider's site that was all over the board.
It peaked at 2.4Mbits but average was 288Kbps. Thing is, their traceroute showed the server is in the same city as me.
As for firewall/etc, not a problem here. Checked and it was not logging any issues. (firewall logs)
The gog.com downloader is slow again, 29KBps... Their only advice is to pause it several times. So I'm stuck babysitting this download yet again. (day 4 now)
It seems like they have some kind of session security on their downloads that is causing the problem. If I click the link and download in under 30 seconds, I can get maybe 5-20 minutes of speed with 5 parts downloading. (200KBps, sometimes burst to 300) But then it will disconnect one of the parts and after that, all requests give a 403 forbidden error. I have to get a new link from "my account" page.
HTTP/1.1 403 Forbidden
Connection: close
Content-Type: text/html
Content-Length: 345
Date: Mon, 29 Jun 2009 05:01:36 GMT
Server: bit_asic/3.3/r5lx2-bitcast-a
Error requesting file.
New attempt in 5 seconds
Attempt 16 of 999 ...
Again, this isn't my end. Their server is closing down my connections and then forbidding me to download. Each time you click the download link, a new encoded url is given. No idea why they are doing it to me. Sure would be nice if they didn't invalidate the links so quickly.
UPDATE AGAIN: Pausing the gog.com download is of no help. I got 23% and then got this:
Whoops: There was an error requesting the file! All transfers stopped, sry :( [E1004]