In the past week I've had a base Impala with the 2.5L while I was in Detroit. Just yesterday, the P5 decided it was going to have some electrical issues so I got a LaCrosse as a rental with the 3.6L
The Impala was competent. Plenty of room and adequate power. Not going to set any records with the 2.5L. It was good around town and could keep up with traffic on the freeway.
Around town, the 2.5 had auto stop technology. If the car was warm enough, the engine would stop at a traffic light until you released the brake.
I didn't care for the sight lines, but I don't think any other car these days is much better.
The LaCrosse obviously is much more luxurious. The 3.6L V6 is powerful and doesn't have the stop/start feature. (Or Enterprise turned it off.)
I didn't care for the shifter on the Buick. I seemed to always end up in neutral when I was looking for drive.
Both cars had numb steering, but no bad habits.
Neither were my cup of tea. Not bad, not great. Just a car.