The newer the XJS, the more reliable it is likely to be, and the six
will be inherently more reliable than the V12,
which has 2 of just about everything to break. An XJS is definitely not
a Lexus.
That said, to me, an XJS without the V12 isn't really an XJS.
My '84 hasn't been so unreliable as to be truly problematic, but it is
not my daily transportation.
It's been towed twice in the 10 years I've owned it, and most of my
problems seem to occur upon starting it
after it's been sitting for an extended period without being driven (my
bad).
I love the smooth power, and the lack of engine vibration when sitting
at a signal idling at 600 rpm.
I've heard rumors that some parts are becoming unavailable, though, such
as the distributor vacuum advance;
perhaps others can comment on that issue.
Before you buy one, make sure you have a local shop with expertise in
the V12.

These cars really are better used every day, but then that is true of
all cars. If it's your daily driver you are much more likely keep all
the little maintenance items up to date, plus there is less likelihood
of seals drying out, oil sludge accumulating, damp in the electrics,
battery problems, etc.

No reason why a V12 should be any more unreliable than the
XJ40-engined cars. In fact, a well-maintained late V12 is probably a
better bet than an early XJ40. History is everything with these cars.
You need to be buying from someone who has all the paperwork detailing
the work that has been done on the car over the last few years.

That the youngest of these cars is more than 12 years old?

The cars do not have a rear axle as such as they have independent rear
suspension. The classic Jaguar IRS has inboard rear brakes either side
of the differential. They are difficult to get at and can be affected
by oil from a leaky diff.

David Betts (davidb@motorsport.org.uk)
The Classic Car Gallery: http://www.ofoto.com/I.jsp?m=17830847103&n=398038677