The reliability of the IS200 is brilliant. Obviously some people have more problems than others but if it is well looked after then the majority are trouble free.
Common problems -
Inside tyre wear on the front tyres - easily fixable for around £100
Water pump leak - fixed for around £200 i believe
Sticky calipers - can be fixed but a new one is between £150 - £300
Headunits before 2001 facelift (botttom loading CD) have error 3 which makes them uselss for CD's - can be fixed but is expensive and most people go for an aftermarket system.
Bodywork chips fairly easily - cosmetic and can be resprayed
Corrosion on wheels - again cosmetic and can be refurbed for between £40 - £70 a wheel
That is all i can think of, there may be a couple more but as you can see nothing is major and highly expensive.
I have had my IS just over a year and so far i have needed the geometry on the wheels doing and new pads and discs, it has gone through 2 MOT's without fault and runs very smooth.
I would recommend the IS to anyone BUT dont buy one if you want fuel economy and i would also say try and buy one that was registered before March 2001 to avoid the new tax legislation.
pre March 2001 = £210 a year
post March 2001 = £415 (as of 2009) £430 (as of 2010)
If anything above is incorrect i'm sure someone will be able to point it out, if theres anything else i can help with just post up a question.