The NorthStar engine originates from a Lotus racing engine design from the late 80’s. It was equipped on few Corvettes in the early ‘90s, then redesigned by GM to be suitable for use in luxury cars.
With the history lesson over, you need to understand this (racing originating) engine is designed for abuse, in fact a sedate driven car will be prone to issues in time compared with ones driven in a spirited way. The oil consumption is intended by design, the cylinders are loose for high rev operation (is a V8 rev-ing up to 6500 RPM). The other contributing factor to the oil consumption is the carbon accumulation in the cylinders due to sedate driving. It is recommended a wide open throttle be run once in a while for cars driven mostly short trips in urban areas. I have seen a 400K miles NorthStar engine torn apart, and the factory honing is still very visible on the cylinder walls.
Another complain about the engine is the head gasket failure rate, although in reality it's not higher than average. Because the engine is crammed in the engine bay, the repair has to be done with the engine out. This is a very costly repair comparing with other cars (or the value of the car), thus everybody tends to complain about it. Most of the time the owners pass the issue to another unsuspecting buyer, thus the percentage of the cars for sale with problems is very high. Naturally most of the people complaining about this issue are new owners, but the reality is really not as bad as you read about it. Personally I know no car locally that had the HG issue. A contributing factor to this is not the design, but the lack of maintenance, especially cooling system. The DexCool is recommended to be changed every 3-4 years or 100K. In time it becomes acidic and corrodes the gasket. Failure to read the owner’s manual is the cause most of the time.
Oil leaks are indeed common, but no big deal, and are originating from the Lotus design with case halves.
These cars are equipped with state of the art (even for our days) suspension (with active struts), have a state or the art stability system, and technologically are still far superior to most of the cars made today.
Because these cars were $50K new, not many people bought them, and as a result, aftermarket is non-existent, plus these cars were designed to be the best that could be, so even the very few available aftermarket parts are really bad comparing with the far more expensive (and recently discontinued) OEM parts. People tend to let the cars go, just drive it until it dies, and complain about it, since they are not willing to spend money and effort to locate OEM parts. When somebody complains about stability and suspension issues, try to find out the mileage of that car. Most likely is a high mileage car that was recently purchased for a too good to be true price. Not many people are willing to spend close to $1000 for a replacement active strut (and you have 4).
Reading the most frequent complaints for these cars: memory seats/mirrors settings, security system issues, automatic climate control, trunk pull down motor, electronic leveling control and so on… well these are features that most of the other cars don’t have.
The other problem is these cars are unlike any other GM car, so it is difficult to locate GM dealers that know how to service these cars properly. A lot of the time, a small bad repair causes bigger issues soon. I am an enthusiast (you couldn’t tell) and I am shocked to find out that I know more about these cars than the qualified GM technicians.