I purchased my '01 SES 24v in September of '02. I have had amazingly trouble-free miles (148k currently!). I upgraded the springs/struts (that was a personal choice), but other than that, nothing major has gone wrong. The key to these cars -- any car, for that matter -- is to perform maintenance on schedule religiously. I have reduced the transmission fluid interval to 20k, as these transmissions are a known weak spot. Around town, I drive this car with the transmission in Drive, not Overdrive, and that seems to have made a big difference in the performance of the transmission. The Duratec motor (24v DOHC) is far superior to the Vulcan (12v SOHC) in both performance and mpg.
24-valve Duratec engine, exterior design (especially with the trunk spoiler), amazing reliability, durability of the paint (though mine gets frequent detailings). Also love the 21+ mpg around town (at 148k on the odometer, no less!). Even better, it starts when I turn the key!
4-wheel disc brakes, better quality plastics on the interior, dual-exhaust setup with bumper cutouts, upgraded leather quality, gauge package instead of idiot lights, bigger boost in power from the V6, 5- or 6-speed auto or manual transmission would be a nice option (as it is on the 2010 model)