I've only had my STS for two months. I read the reviews here before buying it and I am very pleased with my decision to buy the 2006 STS. It is more of a sports car than I expected. The V6 engine has plenty of power for the car. I got it loaded and I am enjoying it very much. I think any complaints you may read here are overblown. You will be glad you bought this car. I just can't comment about the long term reliability because I have only owned it for two months. Its fuel economy has been 23 mpg highway / 14 mpg city.
I love not having to deal with a key! Without touching the remote in my pocket, I can step into the previously locked car, start it, arrive at my destination and hop out. It locks itself! Also, the rain-sense wipers are wonderful. The handling is unbelievable too. It is very, very stable.
I'm not an automobile designer. My suggested improvements sound like complaints and I don't have any. Everyone carries on about the glove box. The car is short on storage space, but what do you need to store in your car? It has space for ink pens, registration, insurance, a tire gauge, etc. What do you want?