This is my second Bonneville. I had a 2000 SE for about 2 years and put 36,000km trouble free on it, except for the Goodyear tires. I traded the 2000 in on a 2001 SSEi and now have 102,000 km. on it.
I found the finish on both cars to be outstanding - the doors, windows, trim, dash, seats - were all excellent. The heating/ventilation system, stereo, and traction control all work flawlessly.
The seats are excellent for long trips and the inflatable lumbar support makes the SSEi the best car for seating I have ever driven.
Both the supercharged and non-supercharged engines are smooth, powerful, and have been completely trouble free. They are quite economical on the highway, if not pushed and driven reasonably.
Once I replaced the Goodyear Eagles with Michelin tires, all vibration disappeared. The 2001 SSEi still has lots left on its brakes and I have had none of the problems other drivers mentioned about steering, belts, loss of coolant, etc.
My only complaint is an occasional misfire that sends the tachometer to zero rpm. The engine does not stall. This started about 20,000 km ago and I was told that the plugs should be changed around 80-100,000km instead of the 160,000km suggested by the manufacturer.
A software update at about the same mileage keeps the transmission engaged in top gear lock-up too long and can produce some "hunting" as it can't decide whether to come out of lock-up or not. Once you get used to it, it is not a problem and it did add to the fuel economy.
The 2001SSEi is the best highway car I have ever driven, however, it is thirsty if pushed. I have had it four years and, other than routine maintenance and a set of new tires at 90,000km, it hasn't cost me a cent!!