I was also having days of trouble with this, but I can connect consistently now. Here's what worked for me on Win7-64:
Get yourself to a fresh start (uninstall drivers and Kies):
1) with your phone plugged in and USB debugging OFF (in my unprofessional experience, Kies will never connect if your phone's usb debugging is enabled), go to your Windows Device Manager. You should see either yellow bangs because your phone's drivers aren't installed correctly, or you'll have "SAMSUNG Mobile USB Composite Device" under USB controllers. Highlight it, click Properties, Driver tab, click Uninstall. Delete the device if it's still visible.
2) unplug your phone
3) If you're using the most recent version of Kies, I recommend downgrading to 2.0.2 because I read other forum posts about complaints with 2.0.3. Go to Windows Start - All Programs - Samsung - Kies - Uninstall Kies.
Now install stuff fresh
5) install Kies 2.0.2. I just googled it. Start Kies. Do NOT update it when you get the popup asking.
6) plug in your SGS2 (remember, USB debugging = OFF). You'll see a lot of typical driver installation bubbles. Go back to your Device Manager and confirm that you have no yellow bangs and you do see your SAMSUNG device under USB controllers.
At this point, Kies should already be connecting. I hope it works for you; it did me.
As an aside, Kies sucks. It's good to prove you have connectivity, install drivers, and to flash your firmware. If you actually want to sync Outlook contacts, Outlook calendar, and music, use MyPhoneExplorer (which btw requires USB debugging ON).