Came from a '03 Tahoe in to my '11 JGC Overland.
Some initial, somewhat random thoughts:
The Jeep's interior wins hands down. Some of that is due to the bells and whistles now available (GPS, satellite radio, voice commands, etc.) but a lot of it is just tactile look and feel.
We got the V6, and it gets 23-24 MPG on the highway. I think our Tahoe once got 16 with a tail wind, going downhill.
Power is comparable. I didn't drive the V8 JGC, and I am sure I would have liked it, but the V6 is perfectly fine.
I miss the all terrain tires I had on the Tahoe. The Goodyears that came with the Jeep suck. They're really nice on the highway, but awful in sand, and slightly sketchy in snow. I can't wait for them to wear enough to be replaced.
I do NOT miss the oil changes on the Tahoe. The Jeep is one of the easiest vehicles on which I've ever changed the oil. The Tahoe was always a mess.
Overall - we love our Jeep. This is our first Jeep product, and we have been very impressed. I always felt like the Tahoe was too big. For me, the Jeep is "just right" sized. I hope I get a good, trouble free 140K+ out of it like we did the Tahoe.