The biggest motivation for passing the IR first time is the cost of failing!! Retaking the test can be a few thousand pounds if you consider re-training etc. I think a Seneca rents for about £350/hr wet incl. instructor in the UK at the moment.
Lots of people don't pass their IR tests first time, so it is not a disaster if you happen to screw up a little. It is however nice to be able to state 'first time pass' on your CV as it is a difficult test, it shows that you know your stuff.
I trained at Aeros quite a few years ago (a very good school), their latest test results are shown below. First time pass on the IR is only 50%, but I think that is about average in the UK.
Aeros - Flight Training at Gloucestershire and Filton
I suggest you have a read of this for the test:
http://www.caa.co.uk/docs/33/SRG_FCL_01.PDF