Personally, i think its a waste of money, you can buy cheaper versions that do the same thing. if you have a "big" problem that urgently needs looking into, such as a P2002 code on an IS220d you need to take it to Lexus, the OBD on the car tells you if there is any problems anyway.
It is nice to know what needs replacing, not much goes wrong with the IS200s,
Ball joints wear out, Rear calipers seize, oxy sensors go, keep on top of that and it'll run like brand new, also make sure your coolant is topped up, simple stuff.
If you really feel the need for one id recommend one from Mac, £50.