I was wondering if anybody owns a Buick Rivera because I recently picked up one.

I haven't had one myself, but my sister had a 1995 Riviera Supercharged for 46,xxx miles (bought it with 135k, and sold it with 181k). Was a very reliable car, with the exception of the harmonic balancer and an idler pulley. I personally would love to have one of these cars one day, as they are very nice riding, and the body has some very stylish lines to it.
My cousin also owns (currently) a 1996, which has the Series II Supercharged, and his only problem mechanically was an idler pulley. He has had his for over 2 years and uses it on his sales route which is over 150 miles per day.
Very nice cars. And with the 3800 you are gaurunteed many miles of driving pleasure.