I'm thinking about buying an 2004 - up TL and I'm just wondering how long these cars usually last. I ask because I see a lot of good deals on TL's but they all have around 80 - 100k miles on them. Thanks in advance.

If you treat them well, they'll last surprisingly long.

The best advice I can offer is to get information about the seller/previous driver(s)/vin history etc..
How was it driven? Was it maintained? Accidents? << the easy questions.

My family bought a 2nd gen (1999) the year it came out and we all took turns driving it up until about 2005 (mostly parents.) They ran the car up past 100,000 miles, were offered decent trade-in value, and decided to upgrade to the 3rd generation since they liked it so much -- having a troublefree, luxurious, speedy ride is quite a departure for them. They've had the '04 since 2005 and no engine or tranny problems at all. Regular maintenance (interior and out) definitely help, and yes I had to use Wikipedia :p

Get an extended warranty! The transmissions are crap! Other than that they are wonderful cars and can last well over 250K will regular maintenance, not just oil changes.