Folks, there's nothing wrong with your phones, cables, USB ports or in most cases your drivers either. The problem is entirely the Samsung (Kies) software. After you've tried all of the forums, that is all of the well-intentioned advice from everyone, you'll come to the conclusion that the software does not work. End of story.
There are people who are using Kies without a problem. I suspect that they're using an older version of Kies or the phones with which they're using it are older
I have uninstalled (cleaned the Registry) and replaced all of the drivers and software (including updating as well as rolling back .NET Framework 4) repeatedly for the last three-four months. Prior to that, both of my phones connected just fine. Then came the last Kies "update" and it now won't recognize either phone. I have also tried installing nothing but Kies on a "virgin" Windows 7-32, a Windows 7-64 and a Windows XP machine all of which were updated with the latest operating systems and drivers with no anti-virus or firewall active. (I have limitless access to brand new PC's) None of this made any difference. A couple of times I got an initial very slow connect, but Kies hung while trying to read the phones' data or it would connect once and never again. Once connected, it would hang when I tried to back up or sync. The only way that I can transfer files to/from the phones is to tether the phones directly through USB and copy/paste files directly. For that you have to know which files not to touch. But the fact that I could do that proves that there is no USB port, cable or driver issue. If the software (Kies) can't talk through the existing driver(s), it's not the drivers' fault. Besides, I've tried quite a few different drivers...
My best advice: Wait till the next release of Kies or some other software that will replace Kies
My conclusion: Samsung makes pretty good electronics, but they (and Bada) completely suck at software