The problem is primarily that the device drivers are a bit buggy, and that is being kind. What I do is run Kies, and tell it to diagnose problems, and it will then reinstall the drivers. Now, here is the trick, once you connect your phone, and Windows installs the drivers(nice and fresh as if you never tried it before), it WILL work. The problem is that once you unplug your phone, on SOME systems, the problem comes right back, and requires using Kies to diagnose connection problems.
Now, I have tested this on a couple of systems, one is a laptop with a NVIDIA chipset and that one ALWAYS has the problem. My other machine has an AMD chipset on the motherboard, and it does NOT have the problem. This implies that there is something about the USB controller that isn't working well with the Samsung drivers. There is a good reason NVIDIA got out of the chipset business, and that is just one part of it.