I have owned an 03 CTS for about 3 years, so obviously I got it used.
It is rear wheel drive and doesn't like snow at all. I live in central pa. Every year it gets tested in adverse conditions. If you live in a warm climate that rarely or never sees snow/ice then sure this point doesn't really apply.
I had it leak oil through the valve cover gaskets. Not a cheap fix. At the time I had to wait 2 months for the actual gaskets to be manufactured because this has been an issue with this motor/year of vehicle. Because of the leak, I had to replace the crank position sensor which was soaked/ruined by all of the oil. The total repair was about $1100 w/parts and labor.
I do my own oil change, tire rotate, brakes, fluid updates and what not. I haven't seen a haynes or chiltons manual for these cars to assist beyond the regular maint. stuff. I'm not saying it isn't out there but I haven't seen it because the majority of the people that own these cars don't do there own auto work. Another thing to mention is it seems as though because of the cadillac badging on the vehicle, parts are more expensive. Keep it in mind when doing maint. .
It's a nice car inside. I love the leather, heated seats, and the fact that the heat and a/c doesn't take long to get going. It's an easy car to put a remote start/keyless/alarm into because of all of the data modules available for gm cars.
The motor has some $hit and git to it. I like it on the highway whether traffic is at 80 mph or I just need to pass someone, it hasn't let me down there.
Suspension makes for a nice ride. It is quite quiet too.
Overall I wouldn't buy another 03 cts. I would go newer, even 04 would be alright. The 03 was the first year and definitly all of the bugs were not worked out.
Do some more research and see if you can find any more motor issues with the 03. I have heard of the same problem I had on more than 1 occasion.
Good Luck