The GTIs come with basic bluetooth. With this, you'll be able to pair your phone to the car so you can:
1. Answer phone calls by pressing the phone button on the steering wheel (or have a passenger answer them from the phone button on the ceiling). You can then press the button again to end the call. The stereo mutes whatever media you're playing so you can hear the call, then unmutes when the call is over.
2. Place a phone call through your phone, then talk over the bluetooth system (you'll have to dial the number on your phone and hit send, but then the call transfers over to the bluetooth connection for hands free talking). At the end, you can press the phone button on the car to hang up.
3. Stream music to your car radio from your phone (if it has music on it)... I have an iphone 3g, and this works really well. I'll check today to see how well the streaming works from some of the apps (like Pandora).
***edit: Pandora streams fine through the bluetooth connection (9w2)***
When you have the enhanced bluetooth, you can do all of the above* and a few extras:
4. Sync your phones contact list to the car. This way, you can use the stereo (and I think the MFD) to place calls without ever picking up your phone.
5. Store favorite phone numbers to your stereo (at least on the nav. I think both though). Think of this as the phone equivalent of presets on your radio.
6. Dial numbers with the car's touchscreen.
7. Caller ID on the touchscreen (At least, the manual says the number of incoming calls is listed), and other normal phone features (recent calls, missed calls, etc...).
GTI's get the basic bluetooth (9w2). You can get the enhanced bluetooth (9w3) aftermarket if you so choose (I think their are links somewhere to where you can buy it, installation guides, etc...). If I remember correctly, it's $500ish for the 9w3, and the install is not difficult. I can't remember if you need the vag com to reprogram some stuff in the car though. I'm sure the how to's will tell you though.
*Because of the altered connection protocol, the streaming music from your phone will not work with the iphone and the enhanced bluetooth (9w3). Everything else, I am told, works dandy.
Personally, I just don't use my phone enough to warrant the upgrade. I like to stream the music, and answer the phone. I might place one or two calls a week from the car, and am happy to do so at a stoplight.