Overall they're surprisingly reliable in 3.0 form: as long as you buy a good example you should be fine. Due to the type of owners they attract, there's quite a lot of good ones out there so don't be tempted to skimp and get a cheap one, the money you'll save will be insignifigant compared to the bills a neglected one can throw up.
When I got mine I checked the following:
Air suspension - make sure there's no faults flagged up on the dash, and make sure it levels correctly. Can be very expensive to fix if broken
Gearbox - check it changes gear smoothly, especially when cold, and make sure there's no strange noises as a replacement gearbox can be a good couple of thousand
Electronics - nearly everything is electric so you need to make sure it all works. You'll feel like a bit of a tit pressing every button in front of the seller, but if a couple of things don't work the total cost can be rather large.
The engine is fairly bulletproof so other than the things I've suggested, just check the kind of stuff you'd check with any car.
Basically, get a well cared for example with full Jaguar / Jaguar specialist service history and you'll probably be fine