Have owned the Sierra for 8 months now and I really enjoy driving this truck.
I get pretty much 20-21 MPG regardless of city or highway driving.
On long stretches of country driving (45-50 MPH, no stopping) actually get about 26 MPG.
Just don't drive that way much.
Inside is big and comfortable.
My kids like it and so does my wife.
Black on black looks awesome.
It is fun to drive.
Pretty good in city traffic, awesome on the highway.
The truck is finished nicely (not like a luxury car), but very comfortable.
Love the touchscreen XM radio with backup camera.
Can't imagine driving this without the camera.
First time with blue tooth for my phone and really like it too.
Have to warn folks that everyone in the car can hear sometimes though!
Would recommend two primary improvements:
Add the higher end luxury touches like heated mirrors, seat memory, heated seats, etc.
You are charging a lot for this truck - add the little things.
Upgrade the battery to lithium ion and allow more all electric use.
Currently engine must come on over 30 MPG.
Lots of light city driving up to 35 or 40 should be allowed.