Sad to see many posters trotting out old reliability myths.
Jaguar have topped JD Power Satisfaction rankings, and many other rankings, on and off for years now. The unreliable ones you're talking about were made in the 70s and 80s by, effectively, British Leyland.
Things looked up in the early 90s when Ford took over. They started bringing modernised toolsets to the construction process, and as a result reliability started climbing. It has continued climbing until it is now well ahead of <a href="http://www.google.co.uk/search?q=mercedes%20reliability&ie=utf-8&oe=utf-8">Mercedes</a>, for example, which is trotted out often as some form of reliability paragon.
It takes a long time to change reputation, that's the problem. That reputation didn't match reality as of about 1995 onwards (possibly slightly earlier) with the dumping of the XJ40 and the move to the X300 design (still marketed as XJ6/XJ8), but people still trot out what they once heard in a bar or from their dad. It's annoying - drop it. Jaguars are as reliable, if not more so, as anything else in their class.
Personally I've owned XJ40 and X300-type XJ6 cars (one a Sovereign, one an XJR). I've owned an X-Type and an S-Type, and am currently contemplating an older XF. During the same time period a friend of mine has owned BMWs and Audis - we've spent about the same on garage bills (an RS8 being a notable exception - bills dwarfed anything I'd seen on the Jags). The X and the S were fine, the XJ40 electrically temperamental, the X300 (XJR) was just superb.
Cheers,
Ian