Muffler fell off and was dragging on highway (this was exciting), apparently rear tailpipe mount was missing and had been wired back on. A difficult repair chore (had to saw off broken end with thick welds, and replace mount).
Hard starting, very long cranking -- in-tank fuel pump was not "priming" on initial cranking. Eventually traced it down to fuel pump "start" relay (in right front fender) - just a $5 part.
Ongoing issue with defective fuel injectors (see online info from Linder and GM factory service bulletins). Windings short out with alcohol in fuel (mandatory in our emissions area), or maybe just with age. New Buick injectors are expensive and are no longer warrantied past 1 yr (according to dealer). But found slightly different replacement injectors with larger windings are available from several sources (got a whole set at a salvage yard) ; no severe missing or dying out since. (previous GM car used Bosch injectors; no problems in 210,000 miles -- but GM thought they could do it better?)
About 95,000 miles; went to replace timing chain (it was sloppy). Timing chain replacement was straightforward, but managed to bend up harmonic balancer sensor rings getting it out (no provision made for attaching any kind of puller, and no room to work -- thanks, GM). Put replacement harmonic balancer in, front shaft started to leak (badly). Replaced front seal 3 times (and changed brands) before I found one that would hold.
Meanwhile, all this seal changing caused some damage to front of oil pan seal, so it seeps some oil (but it's a real bear to get oil pan off, not like Buick 3.8, so it's just going to have to keep seeping...)
Recently replaced spark plug wires and spark plugs -- changing rear plugs and wires is a chore. Have to disconnect and remove top front motor mount (yes!) to remove channel for front spark plug wires (?).
And watch tire sizes -- front tires not a problem, but if you go even one size larger on rear tires, this inane "shield" thingie in left rear wheel-well sticks out and rubs on tire.
Buick power windows issue -- well known from other web sites; got to where driver's window was taking 10-15 minutes to close. Only a complete window assembly is available from Buick, an expensive repair option. But recently, a window-motor-only kit has become available. One must do the usual "beat-n-drill-the-rivets-out" 2-hr exercise, but certainly worth the $400 savings...
Brake wear issue: For some obscure reason, when the brake pads get down to the "wear warning" tab, the inner pad has usually already failed and eaten the brake rotor. Replaced 3 rotors so far (not always on the same side). Have also installed rebuilt calipers, new slider hardware and new brake hoses, but the problem persists (check those brakes every spring and fall!). Get "lifetime replacement" brake pads. Could be the brake pads are at fault, I don't know.
Also (last fall) needed to replace a suddenly-leaking heater core -- this was "fun".
3.3 engine is a strong runner, and gives some serious competition to the former 3.8 engine (perhaps even a bit smoother). Always starts and runs in any weather (assuming fuel injectors haven't shorted out).
Cooling system and electrical system have been relatively trouble-free.
3-Sp. auto transmission is not as efficient mileage-wise as the 4-Sp in the Olds (22-23 mpg in mixed driving), but hopefully it will be less prone to "sudden input sprag failures in the night" & suchlike (thanks, GM).
Brakes are efficient and effective. (wife reports several panic situations where the previous car "would never have stopped in time")
The trunk is roomy (though not a flat floor) -- back seat is somewhat cramped (very difficult to get some types of child seats fastened in, for instance).
Front seats are comfortable & adjustable, though have never thought the "seat-belt-on-the-door" was a good idea (this also causes blind spots in side visibility).
Air conditioning has worked well.
Space for installation of radio or stereo is unusually short and cramped, and seems to get very hot -- not sure what that's all about.
Handling is fairly "sure" and steady, I think I had to replace one CV shaft, but no other front end work.