I have an old diesel GL420(450 in the US) with offroad package (sturdier and locking diffs and etc) from 2006. I believe I've gone through some of the most common problems that there can be, but I'm still driving it on a daily basis and I like it. Currently at 240 000 miles.
Firstly, the issues:
1. Gearbox - it will eventually fail, but isn't a big thing to replace. Bigger thing would be to find a suitable gearbox due to different versions.
2. Air suspension - it will also fail, but if maintained properly and replaced parts are not some Chinese shnit, then this is more of a once in 10 years thing.
3. Tailgate - just clean the locking mechanism and keep the handle clean and you'll be fine. I have never had anything actually broken in it.
4. Yeah, water in the lower compartments (trunk, under the drivers seat, under the passenger seat) - that will be an issue, but it can be prevented by unplugging the rubber plugs from the bottom once a year. Mostly the biggest PITA thing (with offroad package that has full range of undercoverings...). If this is not done, then you'll start having random faults and mysterious things happening with the SAM module and so on... Mysterious battery drainage and so forth...
5. Sunroof - just old age will become it's toll. I had to take the whole headliner apart and few minor things replaced (sunroof motor and cables that move the sunroof). Root cause was just dried out and lack of lubricant in the rails that in turn caused excessive stress on the motor...
6. Well... That is actually it...
Now, the good things =) :
1. The best All-In-One car. I use it to travel (good to sleep at the back for two people - flat floor), goes everywhere and is comfortable at the same time.
2. During summer I tow my cars around quite often and it is a joy to drive it. The fuel consuption is non existent concidering that it is more like a 4x4 than an AWD.
3. It is a seven seater and the 3rd row is not only for children.
4. The 4L diesel engine is bulletproof unlike the 3. something engine. Don't know about the petrol engines, but this 4 liter diesel has enaugh torque to do anything and is as econimical as one could think of. I can overtake cars on the highway and at the same time tow a 3t trailer.
5. I don't understand people complaining about the upkeep - yeah, it isn't cheap compared to some Chevy Spark or a VW Polo, but it isn't even supposed to be comparable with those. If you compare this car with other similar cars like a LandCruiser (not overstatment, but it is as capable as a Toyota LandCruiser) or a Range Rover or Cayanne, then you'll understand that this car is cheap to own.
6. And truly, this car can go anywhere. I didn't find pictures of the car in a knee deep mud or river, but I did find few other pictures of the places I have reached:
I like the car and I am only OK to swap it with next gen GL/GLS with the offroad package. Without the offroad package I would not be interested and I haven't seen any other car as capable as the GL. VW Touareg (a cheap Cayanne) has the same offroad capability probably, but it is much smaller inside.