Just picked up my 07 6 mt. I've never owned a car with MT before, in fact, i've never owned a car. This is my third vehicle, the previous two were Yukon's. I'm young and i bought the G for its great combination of looks, features, performance, and of course price. In my opinion the MT just added a little more to the performance side of things.
Besides the manual being 6 speeds, and the auto 5, just drive the two one after the other and you'll feel the difference, the MT is a much more spirited car. Besides launches and all 0 to times being quicker, the MT puts the control of the heart of the car at your fingertips, which had some trouble finding the paddle shifters in the AT.
I've heard some people complain about how sticky the clutch is, how rough the takeoffs are, etc... I just wanted to put your mind at ease about this, it was sticky to me, for the first day and a half. Since then I have decided that this may be the best manual transmission i have ever driven. The clutch is tight, and the throws couldn't be shorter. I LOVE IT.
Long and short, if you bought the car for its performance and spirited drive, get the manual. On the other hand if you buy it with the intention of cruising to work, latte in hand, leave the hard to find MT on the lot for someone else.