So far all the negative remarks I've read come from people with the automatic gear box. Now--If, knowing it's a Jag, there's a manual gear box available, and you go for the automatic, I don't have much sympathy for you when you encounter problems. I don't say that to be a jerk, it's just kind of a given. Do your homework before buying a car.
SHOULD the auto transmission be reliable? Of course. It's a shame that Jag hasn't been able to sort out their problems. Hopefully the XF will turn all that around.
A Jag is going to have more 'gremlins' than a German or Japanese car in the same class. If you're a Jag person you're aware of this (and you'd probably have purchased the 3.0 litre manual in the fist place), and you'll (a) make sure you've got a good warranty and/or (b) have the repairs made, satisfied with the fact that while you may not have a mechanically bullet-proof car, you've got better taste than the masses driving their boring, lame Lexus or BMW to work.
Just to reply to your question. My X-Type is a joy to drive and own, but it, like ALL cars, has its faults and foibles. Jaguars seem to suffer a few electrical glitches, but whatever car you own you WILL get the odd problem, especially if you buy a used car, but even new cars need to be ironed-out as it were.
So I think some people seem to want a car that is flawless and without ANY faults... Almost impossible.
A car is also about how it makes you feel, and buying into the heritage of a certain brand. For reliability, buy a Ford or VW, and drive with no imagination nor passion. Dull cars for non petrol-heads.
Buy an Alfa and you'll understand what I mean, they are like women... Beautiful, inspiring, fun to live with, sexy... But expensive, and can break your heart and empty your wallet.