Indeed, you can't beat a GM vehicle with the Series II 3800. Some engines don't survive the inevitable intake manifold gaskets going bad (they all go bad eventually), which will dilute the oil with coolant in many cases, trashing the bottom end. If it survives that (the new gaskets are an improved design), they are bullet-proof, powerful, simple cast iron motors that will run to 300k and beyond very inexpensively. 30 mpg on the highway is not unheard of, but 26mpg combined hwy/city is pretty typical.
If I were buying a 10+ year old vehicle, a 3800 based powertrain would be my first choice, extremely dependable. The 4T65E transmission that is usually paired with them can be finnicky; it's a robust design, but there's a solenoid that likes to malfunction which causes hard shifts. The PCM sees that shifts are taking too long, and it maximizes the fluid pressure to shorten shift time (to prevent excessive wear to the clutches and such), which causes the hard, abrupt shifts. More often than not, a $40 shift kit is a permanent band-aid for this situation, and the transmissions will easily go 200k+.
Grand Prix, Bonneville, Buick Regal, Le Sabre, Riviera, Park Avenue, some Monte Carlos, Oldsmobile 88, are some of the vehicles you'll find it in. Some Camaro/Firebird models will have a longitudinally mounted 3800.
Series III motors came out in 2004 and beyond, and offered many improvements. I had a 2001 supercharged Bonneville with the Series II engine, and it was the most fun, dependable car I've ever owned. I had absolutely no qualms about driving 2,500 miles across country, even with 175k miles. Lots of little stuff liked to break (window regulators and misc items), but nothing that would keep the car from getting from point A to B. Parts were dirt cheap, and vehicle was incredibly easy to work on.
