I owned one, an earlier one, but I think the issues I had with mine were fairly common throughout.
Wishbones - They're bloody expensive to replace, and there's little to no pattern parts to get hold of. Any knocking/banging in the suspension and be suspicious of the rubber/metal (metalastic?) bushes being knackered. I had one rear lower wishbone replaced at a garage, and that was 900 quid. I ended up getting second had parts to keep mine on the road in it's last year.
I had the 3.0 engine, and it was great. Never experienced a 4.0, but I believe the early 4.0 had issues around the nikasil liners breaking down and becoming porous, later 4.0 were sorted (so I guess by 06 it was fine.
The gearbox on mine packed in just at the point I sold it - with under 100,000 miles (around 80 odd I think).
Battery is in the boot, and my boot seals went leaky (check the boot for condensation and then lift the carpets to look at the spare tyre - if there's water down there then it's obviously bad news.
Electric windows are a known weakness. Replacing the mechanism is around 80-100 quid.
Exhaust - the hangers at the rear of the car are known to rust through. You end up with a very wobbly rear section. I repaired this by bolting some makeshift brackets in place. Repairing with genuine parts would be a pain (as I think the bumper has to come off and a large bar replaced at the back - it was a while ago, sorry for being a bit hazy).
Rear calipers were sticky on mine - and I'd find out when the pads wore out very quickly (thankfully they're cheap to replace - the calipers, much less so).
Air-Con/Heater. The wiring for the heater/air-con is known to break down as the valve which controlled whether to send hot air or cold air into the cabin is at a pretty exposed place somewhere low down around the front wing (from memory). The failsafe position for this is to send hot air into the cabin. Again - perhaps a simple repair, but I'm pretty sure I found something that meant it wasn't simple and I put up with it for the six months through the summer and into the winter before I got rid of mine. I'd run thr car till it got hot and then make sure all the heating controls work as expected.
Earlier models had problems with the front light lenses becoming a bit hazy too (I spent a while cutting mine back to clear).
The power column (when you put the key in the steering wheel will move to the position you left it in, and when you take the key out it lifts and retracts out the way to let you get out more easily) died just before I got rid of mine too.
I think they did make quite a few improvements through the production, so an 06 may be considerably better than the earlier model I had. When I was looking at them though I saw quite a difference in trim levels, from basic to fully loaded, and in a basic model you will notice things missing.
All that said, when I had mine, I loved it. It was powerful (in a surging wafty kind of way), the ride was brilliant and it was really comfortable. Cabin space was considerably smaller than I expected, but I got used to it very quickly and ended up liking it. Things did feel of reasonable quality. When it went wrong, lots of things seemed to go wrong, all quite quickly together - hence I ended up trading mine in.
Hope even a tiny bit of that helps...
