The IS250 doesn't really suffer from any major problems. There is an extensive post on the forum regarding things to look out for -
The short version is - not a lot. As far as ownership is concerned, have a read here
Generic service parts (brakes,filters, etc) are generally available through general motor factors at reasonable prices as is the water pump). However, shock absorbers are Lexus only and not cheap, but do seem to last pretty well. Also, exhaust parts are expensive from a main dealer and there is limited availability from 3rd party suppliers but the exhaust is not a common issue. The spark plugs can be expensive to replace (lots of disassembly to get to them - hence high labour charges) and are due at around 60K miles.
The Mark Levinson audio is excellent but the standard audio system is not too bad either. The SATNAV is just about adequate but I think from 2009 on, the system was updated via a flash drive and I think this is dealer only and not cheap. Rear leg room is also very limited
HTH