1) We have just over 40,000 Miles on our 08 LTZ and have had NO mechanical problems what-so-ever. I have been very impressed with the engine and trans. The only complaint I have is transmission seems to get confused about whether or not to downshift in some passing situations.
2) We get 24-25 mpg consistantly in mixed driving on 87 octane (20% city 80% highway). Last Sepetember we drove form San Antonio, TX to Tucson, AZ to the Grand Canyon and back to Texas. On that trip we got 28-29 mpg. This mileage is based on normal driving doing the speed limit + and using the cruise control. If you drove mileage minded 30+ mpg would be no problem.
3) The Onstar is IMO a personal choice you'll have to make and you'll get a full year to try it out. We down graded to the 'safe and sound' plan after our year was over. The Nav system is pretty good but we didn't use it enough to justify the price.